This study describes the clinical, roentgenologic, and histopathologic studies of pulmonary toxicity induced by treatment with bleomycin. In 168 patients with evaluable cases who were treated with different dose schedules of bleomycin, the overall incidence of pulmonary toxicity was 39.8%. The mean total dose producing pulmonary damage was about 100-150 mg/m2. In 31 patients (46%), pulmonary changes were detected only after systematic auscultation of the lungs (fine crepitant rales at one or both bases). In 36 patients (54%), roentgenograms showed reticular or reticular micronodular lesions located mostly at the lower lung zones and at the costophrenic angle 1-2 weeks after the crepitations were noted. In seven patients postmortem examination revealed hyperplasia and endoalveolar migration of type II pneumocytes and macrophages, fibrinous edema, hyaline membranes, and newly formed reticular and collagen fibers within the alveolar septa. The radiologic and histopathologic findings were consistent with the diagnosis of interstitial pneumonia. These pulmonary reactions represent a delayed dose-dependent bleomycin effect and are probably due to the preferential distribution of the drug in the lung tissue.