Endometriosis is frequently associated with infertility and spontaneous abortus. Studies which had been carried on the subjects with endometriosis revealed the abnormal expression of the antioxidant defense enzymes, and suggested an inbalance in free radical metabolism. This study was designed to determine the activities of superoxide dismutase (SOD) and glutathione peroxidase (GPx) and some of the lipid peroxidation indices (malondialdehid and total sulfhydryl groups) in the eutopic (n=17) and ectopic (n=22) endometrium of the patients (n=22) with endometriosis. Statistical analyses were made using Wilcoxon signed ranks test. Malondialdehyde, total sulfhydryl levels and glutathione peroxidase activity did not differ significantly whereas SOD activity was elevated in the ectopic endometrium (3512±1502 vs 1893±780 U/g protein; p=0.002). Malondialdehyde levels of the ectopic endometrium were significantl y and positively correlated with plasma 17- estradiol concentrations (r=0.683; p=0.001). Neither SOD, nor GPx activities were correlated with plasma estradiol. Since the increment in SOD activity appeared to be independent of the MDA levels, it was suggested that some other factors, such as cytokines and other agents released by activated macrophages, may have an influence on the altered oxidant status of the ectopic endometrial tissue.