We studied the clinical course of eighty-six patients with farmer's lung for a period of 5 years. The patients were first evaluated at an acute or sub-acute stage of the disease and 1, 3, 6, 12 and 60 months thereafter. Special attention was paid to the development of lung function and radiological findings with reference to corticosteroid treatment and antigen contact during the follow-up. Many of the patients were severely ill at the acute stage of the disease. Most of the recovery took place during the first month. Significant improvement of lung function happened up to 6 months, thereafter the improvement was insignificant. After 5 years, respiratory symptoms were reported by 65% and minor respiratory dysfunction (lowered diffusing capacity) was observed in about 40% of the farmers. Thirty-two per cent of the patients showed diffuse opacities in chest X-ray. Corticosteroid treatment had no effect on long term prognosis. There was no difference in the recovery of lung function between those who returned to work compared with those who did not. Exacerbations happened in 8% of the patients during the follow-up. We conclude that respiratory symptoms, minor airway dysfunction and mild radiological fibrosis are common findings after 5 years of an acute or sub-acute stage of the farmer's lung. However, in Finland two-thirds of the patients return back to their previous occupation of farming and cattle feeding.