Twenty-seven patients with open-lung biopsy findings consistent with extrinsic allergic alveolitis (hypersensitivity pneumonitis) were studied. In only one case had the diagnosis been considered prior to the biopsy. Among the 27 patients, a causative exposure was subsequently identified in 10, and no apparent cause could be found in the remaining 17. The prognosis for the entire group was good; all 10 patients in whom an exposure was found and 13 of 17 without causative exposure are alive without progressive lung disease.