Eighty-eight children younger than 15 years with acute lymphoblastic leukemia were treated with two similar protocols. Both regimens consist of multidrug combinations, with CNS prophylaxis. Sixty-four patients were enrolled on first protocol 0171 with vincristine and prednisone induction therapy. After remission induction patients were randomized on two arms: regimen A which consists of intensive courses of methotrexate with vincristine and prednisone reinductions and administration of cyclophosphamide. Regimen B is similar to regimen A, only courses of methotrexate are alternated with courses of 6-mercaptopurine during consolidation and intensification therapy. In maintenance phase both regimens have daily administration of 6-mercaptopurine and twice weekly administration of methotrexate, with vincristine and prednisone reinductions. The second protocol 0276/A is similar to regimen B of protocol 0171, but L-asparaginase is administered in the end of induction phase and total duration of therapy since induction of complete remission is prolonged from 2.5 to 4 years. Complete remission rate ranged from 92 to 96% in the patients achieving complete remission. In protocol 0171 actuarial proportion of children alive in complete remission for more than 10 years is 60% at present. The actual median survival of all children treated with protocol 0171 is 46+ months and actual median duration of complete remission is 43+ months. There was no significant difference between patients treated with methotrexate alone and those treated with combination of methotrexate and 6-mercaptopurine during consolidation and intensification phase. In protocol 0171 the relapse rate was 28% and the death rate in complete remission was 15%. More than 70% of complete responders in protocol 0276 is in continuous complete remission from 2+ to 57+ months, relapse rate is only 4% at present and the death rate in complete remission is 9% in this protocol.