To determine the long-term effects of farmer's lung disease and the factors influening the outcome, 141 patients with farmer's lung disease were evaluated. At the time of the last follow-up, 29 patients had died and 92 (mean age, 54 years) were studied clinically, physiologically, and radiologically. The mean length of disease was 14.8 years (range, 2.25 to 40 years). Symptoms at the time of the last follow-up included complaints of cough (33 per cent of the patients), breathlessness while walking on the level (20 per cent), breathlessness on minor exertion (14 per cent), and breathlessness while at rest (3 per cent). Twenty-eight per cent had chronic bronchitis. Thirty-nine per cent (36 of 92 patients) had some evidence of interstitial changes on roentgenogram. Abnormal vital capacity was present in 11 patients (12 per cent), abnormal total lung capacity in 11 (12 per cent), and abnormal CO difussing capacity in 27 (30 per cent). The ratio of one-sec forced expiratory volume to forced vital capacity was abnormal in 23 patients (25 per cent), and arterial PO2 was abnormal in 39 (40 per cent). Patients with a history of 5 or more symptomatic recurrences had significantly smaller values (P less than 0.05) for vital capacity, total lung capacity, and CO diffusing capacity than did those patients with less than 5 recurrences. There was no significant relation between continued farming or length of disease and lung function. On the basis of several measurements of airway function, 34 of the patients (58 per cent) were found to have some abnormality, It is concluded that symptomatic recurrences may be the most important factor in determining the danger of progressive disease. Persistently positive precipitins were correlated with decreased CO diffusing capacity. Moreover, airway disease is relatively uncommon but does occur, and in some cases it is a possible consequence of farmer's lung disease.