Cigarette smoking has been implicated in the pathogenesis of ischemic heart disease, emphysema, obstructive lung disease and neoplastic disorders. More than 1000 constituents of smoke, including many oxidants, pro-oxidants, free radicals and reducing agents, have been identified. The activities of erythrocyte superoxide dismutase (SOD), catalase (CAT) and glutathione peroxidase (GSH-Px), which are the important components of antioxidant defense system, were measured in 100 healthy volunteers. This study included heavy smokers (consuming cigarettes ≥20 per day; n=30, group I), light smokers (consuming cigarettes< 20 per day; n=30, group II), passive smokers (exposed to cigarette smoke in the indoor environment; n=20, group III), and non-smokers (n=20, the control group). While activities of SOD and CAT in erythrocytes were significantly lower in groups I, II and III than in the control group (p<0.01 for all), mean erythrocyte GSH-Px activity in group III was higher than that in groups I, II and in controls. These results suggest that the increased oxidative stress occurs in smokers, owing to the free radicals present in smoke. It might cause a decrease in antioxidant enzyme activities and oxidant/antioxidant imbalance. We also observed that passive smokers were affected by the environmental smoke to the same extent as active smokers.