Objective To assess the spectrum of causes, clinical features, differences between disease phases, and prognosis of extrinsic allergic alveolitis (EAA).Methods Patients with EAA diagnosed at Peking Union Medical College Hospital from August 1983 to May 2007 were analyzed retrospectively. Their medical records were examined to gather clinical, laboratorial, radiological, and histopathological data. Patients were divided to three phases (acute, subacute, and chronic) according to clinical presentations. Follow-up data regarding treatment response, subsequent radiological and pulmonary function studies, and clinical outcomes were collected.Results A total of 21 cases were cnrolled. Among them, 11 were subacute, 10 were chronic. The most common exposure was pet birds (6 cases, 28.6%). The primary abnormality of pulmonary function was restriction and/or reduction in diffusing capacity (12 cases, 63.2%). The most common findings on high-resolution computed tomography (HRCT) were ground-glass opacities (13 cases, 68.4%) and centrilobular nodules (8 cases, 42.1%). Airway obstruction in pulmonary function test, cmphysema, lung cysts, and fibrosis on HRCT were more frequently seen in chronic than in suhacute patients, though the differences were not statistically significant. Bronchoalveolar iavage fluid (BALF) showed lymphocytosis. The total cell count and the percentage of neutrophils were significantly higher in subacute than in chronic patients (P