In this study of children with acute nonlymphocytic leukemia an attempt was made to prevent central nervous system relapse and to determine whether this therapy, coupled with multiagent chemotherapy, would be successful in prolonging durations of complete remission. Central nervous system relapses were prevented by irradiation, although patients who received this therapy did no better than those who did not receive irradiation. A small group of patients received irradiation to the liver and spleen, but this modality also failed to improve the duration of remission. Control of extramedullary leukemia, in this study, failed to improve remission duration because bone marrow relapse was not prevented or delayed. It is unlikely that focal therapy will have a significant impact in acute nonlymphocytic leukemia until longer marrow remissions are achieved.