The prevalence of farmer's lung disease and antibodies to farmer's lung antigens were studied in a probability sample of over 1,400 farmers in north central Wisconsin. The prevalence rate of farmer's lung disease was 4.2 per 1,000 farmers. The most prevalent antibody type was directed against Micropolyspora faeni, with 6% of the study population precipitin-positive. Prevalence of antibodies to M. faeni was associated with dairy farming, larger farms, and larger dairy herds. An additive effect of hay acreage and size of dairy herd was observed. An independent relationship was also observed for cigarette smoking, with antibodies to M. faeni more prevalent among nonsmokers.