Twenty six children with late stage acute leukemia in relapse were treated with a total of 48 courses of guanazole at a dose of 15 g/m2/dy for 5 days, given approximately every 2 wk. No complete or partial remissions were achieved in 18 evaluable patients, although 2 of 3 evaluable patients with acute myelogenous leukemia and one of 15 evaluable patients with acute lymphoblastic leukemia showed a transient decrease in the number and percentage of blast cells in the bone marrow and peripheral blood. Bone marrow depression, attributed to the guanazole therapy, occurred in all but 6 evaluable patients.