Adult T-cell leukemia/lymphoma (ATLL) is an HTLV-I associated lymphoid malignancy frequently seen in Japan. Abdominal involvement in 40 patients with ATLL were assessed by ultrasonography and the findings seen in four clinical types, acute, chronic, lymphoma and smoldering, were compared. Splenomegaly was frequently found in the cases of acute and lymphoma types, and the sizes of the spleens measured by ultrasonography correlated well with the disease activity. Hepatomegaly was also found more frequently in acute and lymphoma types, and hepatosplenomegaly was proved to be due to the infiltration by ATL cells. Nodular lesions in spleen and liver and abdominal lymph node swelling were also found frequently in the lymphoma type but rarely in the other types. Ascites, pleural effusion, and pericardial effusion were found in the active stage of acute and lymphoma types. Ultransonography also could detect findings associated with therapies. Thus, ultrasonography studies were found to be very useful for assessing the clinical classification, examining various pathological conditions associated with ATLL, and monitoring the disease activity.