A 26-year-old man presented with a 38 degrees C fever of undetermined origin in November 1977. His fever subsided in 1 month, but recurred concomitantly with dysphagia on December 10. For this reason, the patient visited our out patient clinic on December 15, and was hospitalized because a mass was found radiographically in the left lower lung field. A contrast-enhanced computed tomographic scan of the chest revealed an irregularly-shaped mass encircling the esophagus. Based on open-lung biopsy findings, the mass was diagnosed as non-Hodgkin's lymphoma, diffuse pleomorphic type. Neither swelling of the superficial lymph nodes nor hepatosplenomegaly was detected, and a bone-marrow aspiration specimen disclosed no atypical cells. Ga scintigraphy found accumulation only in this region, suggesting the posterior mediastinum as the origin of the lymphoma. Reports of non-Hodgkin's primary lymphoma in the posterior mediastinum are rare. Further, to our knowledge, there have been no reports at all on the diffuse pleomorphic type to date. This case was accordingly considered to be of highly informative value.