The authors investigated the effect of hyperlipoproteinaemia on the formation of free oxygen radicals to which a significant role in the genesis of atherosclerosis is ascribed. They cause among others peroxidation of blood lipids with formation of lipoperoxides with a higher atherogenity. Using the method of luminol-dependent chemiluminiscence, their spontaneous and activated production in phagocytic blood cells was examined in a group of patients investigated on a long-term basis on account of hyperlipoproteinaemia (n = 24). The results were compared with a group of healthy subjects (n = 20); concurrently also blood lipids were examined. The authors revealed a statistically significant reduction of the spontaneous production of free oxygen radicals in the group of patients with hyperlipoproteinaemia. On examination of the activated production the drop of oxygen radical formation was at the borderline of statistical significance.