We examined autonomous and interleukin-2 (IL-2)-responsive growth activities of leukemic cells derived from peripheral blood, as well as several clinical manifestations, including serum lactate dehydrogenase (LDH) level, of 35 patients with adult T-cell leukemia (ATL) to determine whether these properties were related to prognosis. Growth activities were measured by [3H]-thymidine incorporation of the cells after 24 hours' culture with or without exogenous IL-2. Both autonomous and IL-2-responsive growth activities were higher in the patients than in healthy controls and were significantly correlated with each other (P < .0001, r = .956). Both higher growth activities were significantly associated with shorter survival times (P = .0042, r = .472 and P = .0117, r = .421, respectively). An increased serum LDH value was also significantly associated with shorter survival times (P = .0011, r = .530), but corrected calcium level, sex, white blood cell count, or age were not. These results strongly suggest that both growth activities of primary tumor cells, in addition to the serum LDH value, are prognostic determinants in ATL. We propose a new prognostic classification combining LDH values and autonomous growth activity into three groups: (1) high growth activity and high LDH; (2) high growth activity and low LDH, or low growth activity and high LDH; and (3) low growth activity and low LDH, which showed a significant relationship to survival time (P = .0014; the median survival time for each group was 39, 94, and 340 days, respectively).