In connection with its release of earnings on April 28, 2009, the Registrant is making available certain additional information pertaining to its properties and operations as of and for the period ended March 31, 2009. This information is furnished herewith as Exhibit 99.1 and is incorporated herein by reference.

The information included herein, including the exhibits, shall not be deemed "filed" for any purpose, including the purposes of Section 18 of the Securities Exchange Act of 1934, as amended (the "Exchange Act"), or subject to liabilities of that Section. The information included herein, including the exhibits, shall also not be deemed incorporated by reference into any filing under the Securities Act of 1933, as amended (the "Securities Act"), or the Exchange Act regardless of any general incorporation language in such filing.

The Registrant uses non-GAAP financial measures in earnings press releases and information furnished to the Securities and Exchange Commission. The Registrant believes that these measures are helpful to investors in measuring its performance and comparing such performance to other real estate investment trusts ("REITs"). Descriptions of these measures are set forth below.

Diluted earnings per share ("diluted EPS"), excluding gain on early extinguishment of debt

This measure is defined as diluted EPS adjusted to exclude the gain recognized on the repurchase of a portion of the Registrant's outstanding 3.5% Exchangeable Senior Notes. The Registrant believes that this gain is not indicative of normal operations. As such, the Registrant believes that a measure that excludes the gain is a useful supplemental measure in evaluating its operating performance. The Registrant believes that diluted EPS is the most comparable GAAP measure to this measure. A material limitation to this measure is that it does not reflect the effects of the early extinguishment of debt in accordance with GAAP; the Registrant compensates for this limitation by using diluted EPS and then supplementing its evaluation of that measure with the use of the non-GAAP measure.

Funds from operations ("FFO")

Funds from operations ("FFO") is defined as net income computed using GAAP, excluding gains (or losses) from sales of real estate, plus real estate-related depreciation and amortization. Gains from the sale of real estate that are attributable to sales of non-operating properties are included in FFO. Gains from sales of newly-developed properties less accumulated depreciation, if any, required under GAAP are also included in FFO on the basis that development services are the primary revenue generating activity; the Registrant believes that inclusion of these development gains is in accordance with the National Association of Real Estate Investment Trusts' ("NAREIT") definition of FFO, although others may interpret the definition differently.

Accounting for real estate assets using historical cost accounting under GAAP assumes that the value of real estate assets diminishes predictably over time. NAREIT stated in its April 2002 White Paper on Funds from Operations that "since real estate asset values have historically risen or fallen with market conditions, many industry investors have considered presentations of operating results for real estate companies that use historical cost accounting to be insufficient by themselves." As a result, the concept of FFO was created by NAREIT for the REIT industry to "address this problem." The Registrant agrees with the concept of FFO and believes that FFO is useful to management and investors as a supplemental measure of operating performance because, by excluding gains and losses related to sales of previously depreciated operating real

estate properties and excluding real estate-related depreciation and amortization, FFO can help one compare the Registrant's operating performance between periods. In addition, since most equity REITs provide FFO information to the investment community, the Registrant believes that FFO is useful to investors as a supplemental measure for comparing its results to those of other equity REITs. The Registrant believes that net income is the most directly comparable GAAP measure to FFO.

Since FFO excludes certain items includable in net income, reliance on the measure has limitations; management compensates for these limitations by using the measure simply as a supplemental measure that is weighed in the balance with other GAAP and non-GAAP measures. FFO is not necessarily an indication of the Registrant's cash flow available to fund cash needs. Additionally, it should not be used as an alternative to net income when evaluating the Registrant's financial performance or to cash flow from operating, investing and financing activities when evaluating the Registrant's liquidity or ability to make cash distributions or pay debt service. The FFO presented by the Registrant may not be comparable to the FFO presented by other REITs since they may interpret the current NAREIT definition of FFO differently or they may not use the current NAREIT definition of FFO.

Basic FFO available to common share and common unit holders ("Basic FFO")

This measure is FFO adjusted to subtract (1) preferred share dividends,
(2) income attributable to noncontrolling interests through ownership of preferred units in Corporate Office Properties, L.P. (the "Operating Partnership") or interests in other consolidated entities not owned by the Registrant, (3) depreciation and amortization allocable to noncontrolling interests in other consolidated entities, (4) Basic FFO allocable to restricted shares and (5) issuance costs associated with redeemed preferred shares. With these adjustments, Basic FFO represents FFO available to common shareholders and holders of common units in the Operating Partnership ("common units"). Common units are substantially similar to common shares of beneficial interest in the Registrant ("common shares") and are exchangeable into common shares, subject to certain conditions. The Registrant believes that Basic FFO is useful to investors due to the close correlation of common units to common shares. The Registrant believes that net income is the most directly comparable GAAP measure to Basic FFO. Basic FFO has essentially the same limitations as FFO; management compensates for these limitations in essentially the same manner as described above for FFO.

Diluted FFO available to common share and common unit holders ("Diluted FFO")

Diluted FFO is Basic FFO adjusted to add back any changes in Basic FFO that would result from the assumed conversion of securities that are convertible or exchangeable into common shares. However, the computation of Diluted FFO does not assume conversion of securities that are convertible into common shares if the conversion of those securities would increase Diluted FFO per share in a given period. The Registrant believes that Diluted FFO is useful to investors because it is the numerator used to compute Diluted FFO per share, discussed below. In addition, since most equity REITs provide Diluted FFO information to the investment community, the Registrant believes Diluted FFO is a useful supplemental measure for comparing the Registrant to other equity REITs. The Registrant believes that the numerator to diluted EPS is the most directly comparable GAAP measure to Diluted FFO. Since Diluted FFO excludes certain items includable in the numerator to diluted EPS, reliance on the measure has limitations; management compensates for these limitations by using the measure simply as a supplemental measure that is weighed in the balance with other GAAP and non-GAAP measures. Diluted FFO is not necessarily an indication of the Registrant's cash flow available to fund cash needs. Additionally, it should not be used as an alternative to net income when evaluating the Registrant's financial performance or to cash flow from operating, investing and financing activities when evaluating

the Registrant's liquidity or ability to make cash distributions or pay debt service. The Diluted FFO presented by the Registrant may not be comparable to the Diluted FFO presented by other REITs.

Diluted FFO per share

Diluted FFO per share is (1) Diluted FFO divided by (2) the sum of the
(a) weighted average common shares outstanding during a period, (b) weighted average common units outstanding during a period and (c) weighted average number of potential additional common shares that would have been outstanding during a period if other securities that are convertible or exchangeable into common shares were converted or exchanged. However, the computation of Diluted FFO per share does not assume conversion of securities that are convertible into common shares if the conversion of those securities would increase Diluted FFO per share in a given period. The Registrant believes that Diluted FFO per share is useful to investors because it provides investors with a further context for evaluating its FFO results in the same manner that investors use earnings per share in evaluating net income available to common shareholders. In addition, since most equity REITs provide Diluted FFO per share information to the investment community, the Registrant believes Diluted FFO per share is a useful supplemental measure for comparing the Registrant to other equity REITs. The Registrant believes that diluted EPS is the most directly comparable GAAP measure to Diluted FFO per share. Diluted FFO per share has most of the same limitations as Diluted FFO (described below); management compensates for these limitations in essentially the same manner as described below for Diluted FFO.

Diluted FFO available to common share and common unit holders, excluding gain on early extinguishment of debt

This measure is defined as Diluted FFO adjusted to exclude the gain recognized on the repurchase of a portion of the Registrant's outstanding 3.5% Exchangeable Senior Notes. The Registrant believes that this gain is not indicative of normal operations. As such, the Registrant believes that a measure that excludes the gain is a useful supplemental measure in evaluating its operating performance. The Registrant believes that the numerator to diluted EPS is the most directly comparable GAAP measure to this non-GAAP measure. This measure has essentially the same limitations as Diluted FFO, as well as the further limitation of not reflecting the effects of the early extinguishment of debt and preferred share redemption in accordance with GAAP; management compensates for these limitations in essentially the same manner as described above for Diluted FFO.

Diluted FFO per share, excluding gain on early extinguishment of debt

This measure is defined as (1) Diluted FFO available to common share and common unit holders, excluding gain on early extinguishment of debt divided by (2) the sum of the (a) weighted average common shares outstanding during a period,
(b) weighted average common units outstanding during a period and (c) weighted average number of potential additional common shares that would have been outstanding during a period if other securities that are convertible or exchangeable into common shares were converted or exchanged. However, the computation of the denominator for this measure does not assume conversion of securities that are convertible into common shares if the conversion of those securities would increase the measure in a given period. As discussed above, the Registrant believes that the gain on early extinguishment of debt is not indicative of normal operations. As such, the Registrant believes that a measure that excludes this item is a useful supplemental measure in evaluating its operating performance. The Registrant believes that diluted EPS is the most directly comparable GAAP measure. This measure has most of the same limitations as Diluted FFO (described above), as well as the further limitation of not reflecting the effects of the early extinguishment of debt and the preferred share redemption in accordance with GAAP; management compensates for these limitations in

essentially the same manner as described above for Diluted FFO.

Diluted adjusted funds from operations available to common share and common unit holders ("Diluted AFFO")

Diluted AFFO is Diluted FFO adjusted for the following: (1) the elimination of the effect of (a) noncash rental revenues (comprised of straight-line rental adjustments, which includes the amortization of recurring tenant incentives, and amortization of deferred market rental revenue, both of which are described under "Cash NOI" below), (b) amortization of the discount on the Registrant's Exchangeable Senior Notes, net of amounts capitalized, (c) the gain recognized on early extinguishment of debt and (d) accounting charges for original issuance costs associated with redeemed preferred shares; and (2) recurring capital expenditures (defined below). The Registrant believes that Diluted AFFO is an important supplemental measure of liquidity for an equity REIT because it provides management and investors with an indication of its ability to incur and service debt and to fund dividends and other cash needs. In addition, since most equity REITs provide Diluted AFFO information to the investment community, the Registrant believes that Diluted AFFO is a useful supplemental measure for comparing the Registrant to other equity REITs. The Registrant believes that the numerator to diluted EPS is the most directly comparable GAAP measure to Diluted AFFO. Since Diluted AFFO excludes certain items includable in the numerator to diluted EPS, reliance on the measure has limitations; management compensates for these limitations by using the measure simply as a supplemental . . .

(a) Financial Statements of Businesses Acquired

None

(b) Pro Forma Financial Information

None

(c) Shell Company Transactions

None

(d) Exhibits

Exhibit Number                               Exhibit Title

99.1              Supplemental information dated March 31, 2009 for Corporate Office
                  Properties Trust.