The clinical features of 34 Japanese patients with bronchiolitis obliterans organizing pneumonia (BOOP) are discussed. Thirty-two patients (94 percent) had symptoms of cough, fever, or dyspnea. On chest roentgenograms, bilateral patchy infiltrates were seen most frequently in 23 patients (68 percent), followed by small linear opacities in five (15 percent), both patchy infiltrates and reticulonodular opacities in four (12 percent), and reticulonodular opacities in two (6 percent). The bronchoalveolar lavage fluid (BALF) cell findings obtained from 26 patients revealed an increase in the percentage of lymphocytes in 20 patients (77 percent), neutrophils in 15 (58 percent), and eosinophils in 16 (62 percent), and a decrease in the CD4+/CD8+ ratio in 14 of 23 patients (61 percent). Corticosteroids were administered to 25 patients. Except for one patient who died, the prognosis was good in all patients. Further, in patients without corticosteroid therapy, the prognosis was good.