The clinical characteristics of 33 patients with adult T-cell leukemia (ATL) are described. All patients were born and have lived in Miyazaki Prefecture (southwest of Japan). Because of a wide range of clinical presentations and courses, they were subdivided into 2 groups. In the high-risk group, patients presented with high white-cell counts (WBC greater than or equal to 20,000/microliter) and over 30% of abnormal lymphoid cells (18 patients) and hypercalcemia with a low percentage of leukemic cells (5 patients). In this group the median survival time was only 3 months despite various modes of treatment. In contrast, patients of the second group exhibited a low percentage of abnormal lymphoid cells (WBC less than 20,000/microliter and/or leukemic cells less than 30%) and had no hypercalcemia (8 patients). Their clinical course was chronic with a median survival of 8 months, regardless of modalities of treatment. Two patients went through a period when the number of circulating leukemic cells was low (less than or equal to 5%) before overt leukemia appeared. Other clinical features, signs, symptoms, routine laboratory data, serum anti-ATL-associated antibody, cell membrane markers and cytogenetic studies were similar to those observed in other districts of Kyushu island.