Immunological phenotypes of leukemic blasts from 50 children with acute lymphoblastic leukemia (ALL) have been examined with a panel of monoclonal antibodies to evaluate their prognostic significance. Thirty-seven of them were common-ALL positive for CD10 "common-ALL antigen (CALLA)" (NL-1), CD19(B4) and HLA-DR. One was pre-B ALL negative for CALLA and another null-ALL which expressed HLA-DR alone. Six of the remaining 11 cases were traditional T-ALL positive for CD2(9.6), and the other five tentative pre-T ALL positive for CD7(Tp40) but negative for CD2. Twenty-one out of 39 patients with non-T ALL were treated with the standard regimen. The 18 children with non-T ALL having poor prognostic factors, five with pre-T ALL and six with T-ALL were treated with the more intensive regimen. The median follow-up period was 36 (range 4 to 74) months. Their disease-free survival probabilities were compared. It was found that the disease-free survival of non-T ALL patients with poor prognostic factors was comparable to that of the patients without such factors as a result of the more intensive chemotherapy. Among the patients with poor prognostic factors, those with pre-T ALL as well as those with T-ALL, which were positive for CD7 antigen, were found to have significantly short disease-free survival times (P less than 0.03). CD7 antibody is most useful for detecting ALL patients with poor prognoses.