A chromosome study was performed in 18 patients with adult T cell leukemia (ATL), who were divided into three groups according to their clinical manifestations: nine had acute ATL, six had chronic ATL, and three had smoldering ATL. Mitotic cells were obtained from peripheral blood, lymph node, or bone marrow and were analyzed by the G-banding technique. In acute ATL, the chromosome number ranged from diploid or pseudodiploid to hyperdiploid; in the chronic type, from hypodiploid to hyperdiploid; but in the smoldering type, it was diploid. Eight of nine patients with acute ATL had trisomy 3 and/or trisomy 7, whereas none of those with chronic ATL exhibited these aberrations. Patients with smoldering ATL had a normal karyotype. The present findings indicate that the more aggressive the clinical course of ATL, the more complex the numerical and structural chromosome abnormality.