A 69-year-old man was admitted with general fatigue, dyspnea, cough, fever and right pulmonary infiltrations on a chest X-ray films. He had suffered from myocardial infarction 8 years before. Since September 1987, he had been given Aprindine in addition to previous drugs. In early November 1987, he developed dyspnea. Antibiotics were not effective. He was hospitalized on Nov. 13, '87 when crepitations were audible on his right chest. Methylprednisolone pulse therapy was effective, however right pneumothorax developed. He underwent right thoracotomy and lung biopsy. Lung biopsy specimens showed pathological features of bronchiolitis obliterans organizing pneumonia (BOOP). Corticosteroid therapy yielded a remarkable clinical, physiological and roentgenographic recovery. However, approximately two years later during prednisolone maintenance therapy, BOOP recurred. He responded again to corticosteroid treatment, however he died of hepatic failure on Dec. 17, '89. "Idiopathic" rather than "drug induced" was suggested for the cause of BOOP in this case.