Thirty-two children or adolescents had B cell acute lymphocytic leukemia (ALL) diagnosed by demonstration of surface immunoglobulin expression on greater than 10% of their bone marrow blasts. All patients had greater than 25% bone marrow lymphoblasts. Only five of 32 patients (16%) presented with an abdominal mass; however, 24 cases (75%) had FAB L3 morphology. By comparison with findings in common ALL, these 32 children were older (median age, 8 years) and had a higher incidence of central nervous system disease at presentation (22%); all but one were white, and 24 were males. Blast cells from individual cases expressed mu kappa (n = 13), mu lambda (n = 9), gamma kappa (n = 1), alpha kappa (n = 1), or mu with an undetermined light chain (n = 8). The most frequently identified cytogenetic abnormality was the classic B cell-associated t(8;14)(q23;q24) (n = 4); the t(1;19)(q23;p13.3), t(9;22)(q23;q11), and t(1;22) were observed in single cases. Twenty patients were treated uniformly on a single protocol designed for children with advanced B cell malignancy; therapy for the other 12 children varied. Nine children (28%) are surviving event-free; all but one for 3 years or more. We conclude that approximately 25% of children with B cell ALL are curable with intensive multiagent chemotherapy and that classification by immunophenotyping is superior to use of clinical and/or lymphoblast morphologic features.