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Emphysema is one of the most common lung diseases in HIV+individuals. The pathogenesis of HIV-associated emphysema remains unclear; however, radiographic distribution and earlier age of presentation of emphysema in the lungs of HIV+patients are similar to deficiency of α1-antitrypsin (A1AT), a key elastase inhibitor in the lung. Reduced levels of circulating A1AT in HIV+patients suggest a potential mechanism for emphysema development. In the present study we asked if A1AT levels and activity in the bronchoalveolar lavage fluid (BALF) differ in HIV+and HIV−patients with and without emphysema. A1AT levels were measured by ELISA in plasma and BALF from a cohort of 21 HIV+and 29 HIV−patients with or without emphysematous changes on chest CT scan. To analyze A1AT function, we measured elastase activity in the BALF and assessed oxidation and polymerization of A1AT by Western blotting. Total A1AT was increased in the BALF, but not in the plasma, of HIV+compared with HIV−patients, regardless of the presence or absence of emphysema. However, antielastase activity was decreased in BALF from HIV+patients, suggesting impaired A1AT function. Higher levels of the oxidized form of A1AT were detected in BALF from HIV+than HIV−patients, which may account for the decreased antielastase activity. These findings suggest that, in the lungs of HIV+patients, posttranslational modifications of A1AT produce a “functional deficiency” of this critical elastase inhibitor, which may contribute to emphysema development.