Farmers were evaluated for the presence of farmer's lung disease by serologic methods and by clinical histories. From a large farming population screened serologically, 40 of 92 farmers with precipitating antibodies to the thermophilic actinomycetes consented to be evaluated for clinical evidence of farmer's lung disease. Each subject completed a standardized questionnaire which was reviewd by a trained observer. On the basis of the questionnaire and an indepth clinical history, the subjects were grouped into those farmers who had a history of farmer's lung disease (38%), those with no history (50%), or a doubtful group (12%). When these groups were compared for radiologic changes, pulmonary function abnormalities, and immune function, no differences were discernible. None of the parameters tested were useful in predicting which farmer would develop clinical farmer's lung disease. The parameters tested did not provide reliable criteria for differentiating those patients who present without acute symptoms but relate a history of recurrent pulmonary illnesses associated with moldy forage.