A 47-year-old man with dermatomyositis and interstitial pneumonia had been treated with prednisolone since May, 1992, and with azathioprine since April, 1993. During the sixth month of this treatment, primary pulmonary non-Hodgkin's lymphoma (T-cell, diffuse, pleomorphic) developed. Chemotherapy (vincristine and adriamycin) was begun but there was no response. An invasive lesion of the brain was seen on a CT image. Despite cranial radiotherapy, the patient died of respiratory suppression due to progressive brain disease on December 14, 1993. Primary pulmonary non-Hodgkin's lymphoma develops only rarely in patients with dermatomyositis. In this case, oncogenesis may have been related to the use of immunosuppressants.