Over the last three years six patients diagnosed of bronchiolitis obliterans with organizing pneumonia were studied. Diagnosis was established by open lung biopsy in 4 and by transbronchial lung biopsy in 2. The initiation of the symptoms was subacute although one patient evolved to respiratory failure requiring mechanical ventilation. The mean age of presentation was 68 years with male predominance over females of 5:1. The most frequent symptoms were fever and general malaise in 6 patients, cough and dyspnea in 4, respectively and weight loss in 2 patients. Functional respiratory tests showed restrictive ventilation disturbances in 4 out of 5 patients, mixed in 1 and a reduction in diffusion capacity in the 5 patients in whom it was determined. The radiologic pattern of multifocal alveolar infiltration was present in 6 cases. Interstitial involvement was also associated in 3 patients with pleural effusion in 2. Histologic findings of intraluminal polypoid masses affecting the bronchiols and alveolar conducts (bronchiolitis obliterants) with extension to the alveoli forming conjunctive Masson polyps (organizing pneumonia) was found in the 4 patients who underwent open lung biopsy and in 1 diagnosed by transbronchial biopsy although there were quantitative differences in the degree of alveolar involvement. Response to treatment with steroids was favorable in 5 out of 6 patients while the remaining patients spontaneously improved following thoracotomy.