Objective: To determine activity of catalase (CAT) as a antioxidant and carbonic anhydrase (CA) in erythrocytes from acute leukemia cases. Methods: Subjects were recruited from patients attending the outpatient clinics or hospitalised in the ward of the Hematology Department of Yuzuncu Yil University Hospital. Venous blood samples were taken from a total of 67 individuals (31 with acute leukemia and 36 healthy) included in the study. CAT enzyme activity was determined in erythrocytes using Aebi's method and CA by hydration of CO(2). Results: CAT activity was found to be significantly decreased (P<0.001) on average in acute leukemia cases as compared to the control group while erythrocyte CA activity was significantly increased (P<0.001). Conclusions: Our findings point to malfunction of the antioxidant system in acute leukemia patients. Hence we need to investigation the cause and also its possible contribution to prognosis. Furthermore, clarification of the relationship between the antoxidant system and CA inhibitors in the pathogenesis of acute leukemia appears warranted.