Seventy-eight of 362 children with acute lymphocytic leukemia (ALL) had leukemic cells similar in phenotype to normal pre-B cells. When the clinical and laboratory features of patients with pre-B and "null" cell phenotypes of ALL were compared, no significant differences were noted, except that the pre-B cell ALL phenotype had a higher percentage of black children. In contrast, patients with T cell ALL had a higher median age at diagnosis, frequent thymic involvement, and higher WBC counts. Patients with pre-B and "null" cell ALL were treated identically and patients with T cell ALL differently. Although no difference in remission induction rates was noted between patient groups with pre-B and "null" cell ALL, the remissions were of shorter duration for patients with pre-B cell ALL (p = 0.004). Similarly, overt leukemic involvement of both the central nervous system (CNS) and bone marrow was noted sooner in the patient group with pre-B cell ALL. Univariate and multivariate Cox life table regression analyses demonstrate the independent prognostic significance of the pre-B phenotype and illustrate that the prognostic influence of potential relapse risk factors, such as WBC, sex, and age, are specific for leukemia phenotype. These findings may have importance for the design and tailoring of therapy for children with acute leukemia.