Free radicals are highly reactive species that have been implicated in the pathogenesis of many diseases. Reactive oxygen species can initiate lipid peroxidation and DNA damage leading to mutagenesis, carcinogenesis and cell death, if the antioxidant system is impaired. This study was undertaken to examine the prevalence of oxidative stress and the role of antioxidant defence in untreated leukemia patients. The generation of superoxide anion and hydrogen peroxide by leukocytes, plasma malondialdehyde levels, red cell copper zinc superoxide dismutase (Cu-Zn SOD) and glutathione peroxidase (GSH-PX) activities were determined in 30 patients with different types of leukemias prior to therapy. The superoxide anion generation by polymorphonuclear leukocytes was found to be significantly increased in leukemia patients especially those with acute lymphocytic and nonlymphocytic leukemias, while the hydrogen peroxide levels were comparable to the control values. Plasma lipid peroxidation products in untreated leukemia patients were in the normal range. Red cell Cu-Zn SOD and GSH-PX activities were significantly increased and showed no correlation with the hemoglobin content. Although superoxide generation was high, lipid peroxide levels were normal in these patients. This might be due to the increased activities of the antioxidant enzymes (SOD, GSH-PX) which counteract lipid peroxidation. Increased free radical generation, especially superoxide anion in leukemia patients and increased antioxidant defence enzymes, which is an adaptive protective response, are indicative of mild oxidative stress. There were no significant differences for the parameters cited above between different types of leukemias, suggesting that the changes are not specific to the type of leukemia.