Purpose: The aim of our study is the investigation of possible changes in free radical-antioxidant balance in acute leukemias. Material and Methods: In this study, we have investigated 21 acute leukemia patients and 22 healthy volunteers for the control group. Among the acute leukemia patients; 10 of them as AML (Acute Myeloid Leukemia), 8 of them as ALL (Acute Lymphocytic Leukemia), and 3 of them have been diagnosed as MDS (myelodysplastic syndrome). Catalase activity for the antioxidant property and MDA (malondialdehyde) concentration for the free radical level of all collected blood samples were analyzed. Data obtained for each group were analyzed statistically using SPSS-10 software and groups were compared employing Mann-Whitney U Test. Results: The MDA levels in acute leukemia patients have been found to be increased, but the catalase levels found to be significantly decreased compared to controls. Conclusion: Our findings indicate that acute leukemia has increased the free radical and decreased antioxidant levels and as a result free radical-antioxidant balance has been impaired.