Testicular relapse in children with acute nonlymphoblastic leukemia. The clinical course and other distinctive features of five children who developed a testicular relapse 4 months to 25 months after the diagnosis of acute nonlymphoblastic leukemia (ANLL) are described. The chief presenting feature at relapse was painless testicular enlargement, as is also seen in children with acute lymphoblastic leukemia who relapse in the testes. By French-American-British convention, the malignant cells were classified as M4 (myelomonoblastic) in four cases and M2 (myeloblastic) in one. All children received a course of multiagent reinduction chemotherapy and all but one received local irradiation to the testes. Only one of these children, whose relapse was a late event after elective cessation of therapy, is a long-term survivor. A comparison with six previously published cases shows similar clinical characteristics and outcome. Given the poor responses of such patients to conventional treatment, it seems worthwhile to consider the use of intensive reinduction chemotherapy with concomitant bilateral testicular irradiation followed by remission intensification and an autologous or allogenic marrow transplant. 