This study was undertaken to examine the effect of ketosis on the plasma Lipid peroxidation (MDA) levels and erythrocyte glutathione peroxidase (GSH-Px), superoxide dismutase (SOD) activities. Plasma MDA levels and erythrocyte GSH-Px, SOD activities were determined in 17 hyperketonemic type I diabetic patients. The plasma levels of glucose and ketone bodies and other parameters were measured by standart methods. Diabetic patients with total ketone body levels >1mmol/l were considered hyperketonemic. Activities of GSH-Px and SOD, MDA and other parameters levels were measured before and after medical treatment. The results were analysed by Wilcoxon signed rank test. Plasma MDA levels and erythrocyte SOD and GSH-Px activities were significantly higher in the hyperketonemic type I diabetic patients (p<0.001) before treatment when compared following the treatment. In addition low; ketonemia was associated with lower lipid peroxidation levels when lipid peroxidation and ketonemia were determined in the same patients. This study demonstrates that increase in ketosis causes antioxidative enzyme activities to decrease and medical treatment of ketosis can not increase the activities of SOD and GSH-Px. Therefore, ketosis is a risk faktor for elevated lipid peroxidation levels in the tip I diabetes.