The association between leukemic transformation and various features recorded at presentation in patients with refractory anemia with excess of blasts and with or without ringed sideroblasts was analyzed in 255 patients using the proportional hazard model. Features associated with higher transformation rates were: higher values of blasts in peripheral blood or bone marrow; serum haptoglobin; vitamin B12; megakaryocytes in bone marrow; morphological abnormalities in granulo- or megakaryocyte series; male sex; circulating megakaryocytes in peripheral blood; older age; and lower ringed sideroblast proportion. Multivariate analysis was also performed using the following predictor variables: presence or absence of refractory anemia with excess of blasts; sex; abnormal granules in granulocytes; age; and mononuclear large megakaryocytes. Patients were divided arbitrarily into low (hazard ratio, less than 0.45), intermediate (hazard ratio, 0.45-1.85) and high (hazard ratio, greater than 1.85) risk groups. The cumulative leukemia-free rates in the low and intermediate risk groups showed long plateau phases at 95 and 71%, respectively, while in the high risk group, the rate was 10% at 5 years. For clinical purposes, the low risk group should be considered to have nonpreleukemia and the high risk group to have preleukemia.