A possible association between HLA antigens, susceptibility or resistance to leukemia, and responsiveness to treatment has been studied in 144 patients with childhood acute lymphoblastic leukemia (ALL) and compared to other prognostic factors, i.e. white blood cell (WBC) counts, age at onset, sex, ethnic origin, and cell surface markers. All sequentially newly diagnosed children (97) comprised the group for the prospective study (PSG) and were followed for 6 years. The group included 37 patients classified as T-ALL, 41 as CALLA+, 27 as NULL, 12 as B and pre-B, and 27 unclassified patients, who were diagnosed before 1980. During the follow-up period, 45 patients of the PSG died. Forty-seven patients designated long-term survivors (LTS) have been followed 6-20 years after diagnosis, having completed a 3-5 year course of anti-leukemia therapy, and having remained disease free thereafter. High WBC counts at diagnosis and T-cell-surface markers were associated with poor prognosis, as were enthnic origin and specific HLA antigens. Thus, there was one (1) a significant increase in HLA-A30 and a decrease in HLA B-14 in the PSG Jewish patients; and (2) a complete absence of HLA-ALL in LTS while, in the PSG, 8 of 9 HLA-All-positive patients died during the follow-up period. This suggests that HLA-All is associated with poor prognosis in childhood ALL.