Forty-eight children with untreated acute lymphocytic leukemia were evaluated with regard to their clinical presentation and the surface membrane characteristics, mitogen responsiveness, and cytochemical staining features of their lymphoblasts. The presence at diagnosis of 20% or more bone marrow lymphoblasts possessing T-cell surface markers was associated with the development of early relapse and death. Age, sex, initial peripheral leukocyte count (WBC), lymph node enlargement or the presence of hepato- or splenomegaly bore no statistically significant relationship to the patient's lymphoblast type, and only a WBC in excess of 10(5)/microliter correlated with a poor prognosis. Leukemic T-cells were found to be periodic acid-Schiff negative from nearly all patients, helping to distinguish such patients from the larger group of children whose leukemic cells were PAS positive, but negative for T-cell membrane features. In those patients who had multiple relapses, the surface membrane characteristics, staining features, and mitogen responses of their lymphoblasts remained constant. This suggests that relapse occurs in the same clone of malignant cells present at diagnosis and that the above features may be reliably used to evaluate and classify patients beyond the time of diagnosis.