Platelet glutathione peroxidase (GSH-Px) activity was measured in 25 healthy individuals and in 25 patients affected with well-documented coronary artery stenosis. In parallel, malondialdehyde (MDA) production was assayed on platelets following stimulation with sodium arachidonate. Mean platelet GSH-Px activity was significantly reduced and mean platelet MDA production was significantly increased in the patient group. The difference observed in the GSH-Px activity might be a consequence of different nutritional habits, suggesting an impaired intake of selenium, the inorganic moiety of GSH-Px. As low GSH-Px activities in human platelets may be associated with increased MDA and thromboxane production, we hypothesize that a low enzyme activity may be a risk factor for the development of coronary artery disease.