In this prospective study of farmer's lung disease 93 patients were subjected to a follow-up period of an average of 18.6 months. The patients were initially divided into three groups, each receiving corticosteroids either for four weeks, twelve weeks, or not at all, if the disease was less severe. Lung function, clinical findings and chest radiography were recorded at intervals of one, three, six and 12 months and every six months after that. Corticosteroid treatment seemed to have no marked influence on the course of lung function or the prognosis of working capacity. Twelve-week treatment did not produce better results than four-week. However, corticosteroids diminished the appearance of fibrotic changes in chest radiograms. It is concluded that corticosteroids should be given to severely ill patients to ameliorate symptoms, but no apparent benefit is derived from long-term treatment. Eighty-five per cent of patients continued farming and cattle-feeding after recovery. The severity of the initial attack and the occurrence of relapses were found to be the most important factors influencing respiratory performance, subjective symptoms and working capacity in patients with farmer's lung.