Superoxide dismutase activity was determined in leukemic blasts of 24 patients with acute leukemia and was correlated to its type. The activity was significantly lower in five patients with T cell or B cell leukemia, and there was a wide range of distribution in 15 patients with non-T, non-B cell leukemia. Manganese superoxide dismutase activity was significantly lower in all types of acute lymphoblastic leukemia than in normal mononuclear leukocytes. Both total and manganese superoxide dismutase activities in acute myeloblastic leukemia, however, were significantly higher than those in normal polymorphonuclear leukocytes. The prognosis in patients with a low superoxide dismutase activity in leukemic blasts seemed to be poorer. Determination of superoxide dismutase activity in leukemic blasts may have a prognostic value in the management of children with acute leukemia.