Bone marrows of 41 untreated children and adolescents with acute lymphocytic leukemia were studied by combined immunologic and histochemical methods at the time of diagnosis. Eleven were classified as T-cell lymphoblastic leukemias (27%) on the basis of cytochemical stains and E-rosette assay. The patients in this group has low median age of 8 years, relatively low median WBC of 13.4 x 10(3)/cc, 6/11 were female, and only 2/5 males had a mediastinal mass. The girls had a lower median age than boys (7 vs 9 years), none had mediastinal masses or extramedullary involvement, and their survival was greater than 27 months compared to 14 months for the boys (P less than 0.01). All patients were enrolled and treated on the (then) currently active CCSG protocols for ALL. This study emphasizes the fact that not all patients with T-cell ALL have poor prognosis, that sex could be an important factor affecting survival, and that the difference in survival could not be adequately explained by differences in the initial WBC.