Four hundred seventy-one persons associated with farming or dairy production were surveyed by means of a questionnaire for evidence of hypersensitivity lung disease due to inhalation of moldy hay or grain (farmer's lung). Of these, 172 were evaluated with spirometry, serologic studies, and chest roentgenographs. A history typical of the farmer's lung syndrome was given by 14 of the 471 subjects (3.9 per cent). Precipitating antibodies to Micropolyspora faeni were identified in sera from 2 of these 14 subjects (14 per cent), and spirograms were abnormal in 4 (28 per cent). The prevalence of farmer's lung in this community (i.e., 3 per cent) is comparable to that found in epidemiologic studies in Britain and Scotland. Farmer's lung disease may be an important occupational illness of dairy and cattlemen in this country, but the diagnosis may be overlooked frequently because of the lack of patient and/or physician awareness.