Prognostic factors affecting the survival of adult T-cell leukemia (ATL) patients were analyzed in three groups: total cases, leukemia type cases, and lymphoma type cases. Factors found to be important overall, i.e. for total cases, were leukocyte count, ATL cell ratio in the peripheral blood, serum calcium levels and lactate dehydrogenase (LDH) level. Of those, LDH level proved not significant when evaluated separately for leukemia type or lymphoma type cases. Leukocyte count and ATL cell ratio were significant in leukemia type patients, whereas it was serum calcium level that was significant in lymphoma type; there were mutually exclusive sets of factors for the two groups. Thus, prognostic factors for ATL patients should be considered separately for each type of the disease.