The clinical, radiological, physiological and pathological features were studied in 6 cases of idiopathic bronchiolitis obliterans organized pneumonia (BOOP). Patients characteristically had a history of exertional dyspnea, cough, fever, weight loss, bilateral radiographic shadowing, elevated ESR and hypoxemia. Corticosteroids were effective; however, one patient died of cytomegalovirus pneumonia. The pathological features, distinguished by the presence of granulation tissue plugs extending from bronchioles into alveolar ducts and alveoli, are described in detail. The place of BOOP within the broad spectrum of bronchiolitis obliterans between small airways disease and usual interstitial pneumonia is discussed.