Idiopathic BOOP is a clinicopathological entity defined histopathologically by the presence of granulation tissue in bronchioles, alveolar ducts and alveoli. 2 men and 6 women (mean age 47.2 years, range 22-66) with idiopathic BOOP have been observed, the mean period of observation being 31.1 months. In 7 cases the disease started with fever, cough and in 6 with progressive dyspnea. The interval between the onset of symptoms and the beginning of treatment ranged from 2 to 10 months. One woman was asymptomatic. The initial findings were: tachypnoea (6), crackles (7), decreased VC (6), reduced diffusing capacity (6), raised erythrocyte sedimentation rate (7). In 6 cases chest radiographs showed multiple alveolar patchy opacities of various size predominantly in the middle and lower lung zones. HRCT demonstrates peripheral distribution of changes. In one case diffuse reticulonodular infiltrates and in one case solitary tumor-like opacity with cavitation were observed. Seven subjects were given prednisone. Three subjects recovered completely, in 1 clinical and radiological improvement was observed. In 3 cases relapse occurred. The woman with the solitary tumor-like opacity was cured by surgical excision of the lesion.