To identify factors that might improve the prognosis of adult T-cell leukemia/lymphoma (ATL), we reviewed data on 47 autopsied cases of ATL with reference to the complications and cause of death. The primary cause of death was respiratory insufficiency due to pulmonary infection. Respiratory insufficiency was also attributed to the diffuse alveolar damage and pulmonary fibrosis resulting from chemotherapy given and oxygen. About 90% of the cases had infections with one or more pathogens. Cytomegalovirus (CMV) was the most frequent pathogen involved in 35/47 (74.5%) while fungal infections were also commonly seen in 25 of the 47 cases (53.2%). Of these, 17 (70%) had pulmonary aspergillosis. Other neoplasias were present in 10 of the 47 cases, while hypercalcemia was evident in 21 patients. These findings suggest that the prevention and treatment of nosocomial infections and of drug-induced pulmonary toxicity may improve the prognosis and quality of life of ATL patients.