Regular LDL-apheresis treatment of hypercholesterolemic patients has proven to reduce the formation of atherosclerotic lesions. Regarding the underlying mechanisms, cholesterol oxidation products (COP) may play a detrimental role. Therefore, COP levels were determined before and after regular LDL-apheresis treatment in ten patients with familial hypercholesterolemia. - The patients had approximately twofold elevated plasma and LDL COP concentrations on the average as compared to healthy subjects. LDL-apheresis treatment efficiently removed COP from the circulation. As a consequence of a smaller reduction of the COP content (- 52 %) than of the total cholesterol content (-71 %) in LDL, the LDL COP:cholesterol ratio increased. Lipid-soluble antioxidants in the plasma of the hypercholesterolemics decreased to a comparable extent as did plasma lipids. In contrast to nearly stable vitamin C concentrations, plasma selenium concentrations also decreased, resulting altogether in a decreased but still normal serum total antioxidant capacity. - In conclusion, LDL-apheresis treatment effectively reduced potentially atherogenic COP from the plasma. With normal plasma antioxidant concentrations before LDL-apheresis in long-term treated hypercholesterolemics, the observed acute decrease in lipid-soluble antioxidants and selenium by treatment seems not to be as meaningful. The higher LDL COP:cholesterol ratio after treatment needs further elucidation.