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 enrolled.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

emphysema

lung cysts

and fibrosis on HRCT were more frequently seen in chronic than in subacute patients

though the differences were not statistically significant.Bronchoalveolar lavage fluid（BALF） showed lymphocytosis.The total cell count and the percentage of neutrophils were significantly higher in subacute than in chronic patients（P<0.05）.Nonnecrotizing granulomas were seen in 8（47.1%） cases.Improvement or normalization in symptoms

radiography

and pulmonary function test after treatment were seen in all 18 patients with available follow-up data.Five patients recurred.Conclusions The characteristic abnormalities of pulmonary function

findings on HRCT

and pathology are essential for all phases of EAA

and the atypical manifestations such as obstruction and fibrosis can also be present frequently

particularly in chronic cases.Differential cell counts of BALF are related to the phase of the disease.The treatment response and prognosis of EAA are good.