Presenting features of 120 consecutive children with T-cell acute lymphoblastic leukemia (ALL), representing 15% of all patients diagnosed as having ALL during the study period, were analyzed to determine relationships with treatment outcome. Patients' ages ranged from 1.7 to 18.8 years (median, 10.3 years) and their leukocyte counts from 1.7 to 1,070 x 10(9)/L (median, 100 x 10(9)/L). Central nervous system (CNS) leukemia was present in 12.5% of the cases, a mediastinal mass in 61%, and L2 lymphoblast morphology in 32%. A relatively high proportion of cases, 26%, had normal karyotypes at presentation. Of the cases tested, membrane CD1 expression was found in 38% of cases, CD3 in 33%, CD4 in 50%, CD5 in 94%, CD8 in 55%, and CD10 in 35%. Four presenting features were found to confer an increased risk of treatment failure: age greater than or equal to 15 years, L2 lymphoblast morphology, abnormal karyotype, and membrane CD3 expression. This study illustrates the heterogeneity of presentations of childhood T-cell ALL and suggests that the relative importance of risk factors in ALL differs according to immunophenotype and treatment strategy.