Not all farmers exposed to moldy plant materials develop farmer's lung. With the use of a postal questionnaire the occurrence of symptoms suggestive of farmer's lung was studied among the immediate relatives of 37 patients with clinically established farmer's lung, among 36 immediate relatives of the patients' spouses, and among 50 unrelated persons (spouses of relatives). Symptoms of farmer's lung had occurred about twice as frequently among the relatives of farmer's lung patients as among the subjects in the other study groups. The results, supported by findings from earlier studies, imply that genetic factors may be involved in the pathogenesis of farmer's lung.