Farmer's lung is a potentially serious and crippling disease that results from handling a variety of dusty, moldy organic materials. The symptoms are shortness of breath, cough, fever, chills, weight loss, and hemoptysis. This condition differs from silo filler's disease in both causation and symptoms. The roentgenographic findings are those of interstitial pneumonitis. Biopsy of the lung reveals a granulomatous interstitial pneumonitis with epithelioid and giant cells. This report is based on 39 cases. Lung biopsies were done in eight, and two cases are described in detail. Repeated exposure was followed by recurrence. No causative organism was found. A given exposure induced the disease in some persons while leaving others unaffected. Individual hypersensitivity to molds or to their products seemed to be a decisive factor. Antibiotic therapy is not primarily indicated. The essential treatment is for the patient to avoid repetition of the exposure.