Twenty male smokers and 20 non-smokers took either vitamin E capsules (1000 IU/day) or placebo for 14 days. Erythrocytes from smokers showed a marked tendency to peroxidise in vitro compared with non-smokers, an effect which was abolished by vitamin E supplementation. The increased erythrocyte peroxidation may reflect a lipid hydroperoxide-induced decrease in glucose-6-phosphate dehydrogenase activity. Evidence that smokers incurred a sustained oxidant stress also included increased plasma conjugated dienes, decreased plasma vitamin C and an increase in erythrocyte glutathione. Plasma cholesterol, vitamin E and conjugated dienes increased with age in all groups. Results suggest that smokers are under a sustained oxidant stress, some indices of which can be partially ameliored by vitamin E supplementation.