Six years ago "adult T cell leukemia (ATL)" was proposed as a new disease entity of T cell malignancy by K. Takatsuki's research group at the Department of Internal Medicine, Kyoto university (the 17th Annual Meeting of Japan Society of Clinical Immunology, 1976). Thereafter, increased attention has been paid to this disease in Japan because of its characteristic neoplastic cell morphology, clinical and pathologic features and the endemic distribution of patients' birthplace in southwestern Japan. At almost the same time, M. Shimoyarna and Dr. K. Minato at the National Cancer Center confirmed that a majority of T cell malignancies in adult Japanese correspond to ATL (1976). Dr. M. Sasaki at the Osaka Red Cross Hospital presented the first autopsy case of A TL at the 65th Annual Meeting of the Japanese Pathological Society (1976). Later, at the Symposium entitled "Pathology of Non-Hodgkin Lymphoma" of the Japanese Pathological Society (1978), M. Hanaoka of the Institute for Virus Research, Kyoto University, Dr. M. Sakaki and others demonstrated that the pleomorphism of neoplastic cells characterized cytologic and histologic features of A TL based on their observation of 32 cases of ATL. Also at this symposium, histologic studies on A TL by Dr. T. Suchi of the Aichi Cancer Center and the Japanese Lymphoma Study Group resulted in a new category in the classification of non-Hodgkin lymphoma designated "diffuse lymphoma pleomorphic type.