Tests of immune capacity were performed on blood from 49 children with newly diagnosed, untreated acute lymphocytic leukemia, and relation to prognosis was determined. Patients were treated with multiple-drug therapy and prophylactic cranial irradiation. Median follow-up time was 16 mo (range 10--37 mo). Principal unfavorable findings at diagnosis were absolute numbers of T lymphoid cells outside the range 850-- 2500/mul blood, absence of whole blood responses to phytohemagglutinin in vitro, a low titer of complexed antibody, and the presence in serum of free leukemic blast cell membrane antigen. Fourteen patients showed two or more unfavorable findings at diagnosis. Eleven of these have died. Four of the remaining 35 patients have died. A shorter duration of first remission was found among patients with abnormal numbers of T cells at diagnosis. The findings suggest that the immunologic capacity of the patient at diagnosis is an important determinant in responses to therapy.