Although childhood T-cell acute lymphocytic leukemia (T-ALL) and T-cell non-Hodgkin's lymphoma (T-NHL) have certain clinical features in common, T-ALL carries a notably poorer prognosis than does T-NHL. To determine whether the malignant cells from patients with these disorders are distinguishable, we examined bone marrow and/or blood from 51 children with T-ALL and tumor biopsy specimens from 17 with T-NHL, using a panel of monoclonal antibodies directed against T-cell differentiation antigens. We found considerable phenotypic heterogeneity in both T-ALL and T-NHL. Of the T-ALL (defined by >25% blasts in the bone marrow) patients, 33% demonstrated a surface antigen pattern consistent with the earliest thymocyte stage of T-cell development (T9+ and/or T10+, or T6-/T4-/T8-/T3-), 37% were of a midthymocyte stage (T6+, and/or simultaneous expression of T4-helper and T8-suppressor antigens), and 30% expressed surface antigen patterns found on mature thymocytes (T3+, variable expression of other antigens). In contrast, tumor cell phenotypes in the 17 T-NHL patients were approximately equally distributed between mid- and mature thymocyte phenotypes. No NHL samples were classified as the early thymocytic phenotype. Clinical features as related to specific T-ALL immunophenotypes as presented, and the implications of these findings in regard to the current understanding of the differences in tumor biology between T-ALL and T-NHL are discussed.