Reactive oxygen species are generated physiologically in cells with a significant increase in certain pathological conditions, such as inflammation, cancer, aging, degenerative disease. If endogenous antioxidant systems, in our study represented by glutathione peroxidase, are exceeded by this oxidant flux, tissue injury may occur. Activity of glutathione peroxidase (GPx) was determined using Beutler's modified spectrophotometric assay in erythrocytes from autosomal dominant polycystic kidney disease patients. Activity of glutathione peroxidase was significantly (at p < 0.0001) lower there (17.75 +/- 3.69 U/g haemoglobin) compared to the control group (23.26 +/- .61 U/g Hb). Lower antioxidant enzyme defence system of ADPKD patients, here represented by GPx, can potentiate injury caused by free radicals and possibly play a role in the progression of autosomal dominant polycystic kidney disease.