Bleomycin is an antineoplastic, antimicrobial drug originally developed in Japan. A high incidence of pulmonary toxicity in the form of acute, occasionally fatal pulmonary fibrosis has been reported in humans. Toxicity is greatly enhanced when more than 400 mg of the drug is administered. In a 45-year-old woman with metastatic adenocarcinoma, an acute interstitial pulmonary process developed only after 180 mg of bleomycin was administered. The diffuse lung process was suspected from changes in serial pulmonary diffusion capacity for carbon monoxide and forced vital capacity. The chest radiograph remained stable, and the patient had no pulmonary symptoms. The histologic appearance of the lung is presented in detail and the value of serial pulmonary function tests in patients taking bleomycin is stressed.