In order to study effects of cigarette smoking and smoking cessation on plasma constituents and enzyme activities related to oxidative stress, 1255 smokers and 524 healthy non_smokers were investigated in terms of plasma levels of lipoperoxides (LPO), nitric oxide (NO), vitamin C (VC), vitamin E (VE) and β carotene (β CAR). Additionally, erythrocytes were examined to deterimine the level of LPO, the activities of superoxide dismutase (SOD), catalase (CAT) and glutathione peroxidase (GSH_Px). The results showed that, when compared with the average values of the non_smoker group, the average plasma values of LPO, NO and the average erythrocyte value of LPO in the smoker group were significantly increased ( P <0.001), while the average plasma values of VC, VE, β CAR, and the average erythrocyte activities of SOD, CAT, GSH_Px were significantly decreased ( P <0.001). A linear regression and correlation analysis for 65 male smokers who were all 40 years old showed that with longer smoking duration and greater daily smoking quantity, the plasma values of LPO, NO and the erythrocyte value of LPO were elevated, while the plasma values of VC, VE, β CAR and erythrocyte values of SOD, CAT, GSH Px were decreased. In a group of 73 smokers who stopped smoking completely for six months, the average plasma values of LPO, NO and the average erythrocyte value of LPO decreased, although they were still significantly higher than those in the matched non_smoker group ( P <0.05). Additionally, the average plasma values of VC, VE, β CAR and the average erythrocyte values of SOD, CAT, GSH Px increased, although they were still significantly lower than those in the matched non_smoker group ( P <0.05). How ever, after smoking cessation for one year the above average values were not significantly different from those in the matched non_smoker group ( P >0.05). This finding indicates that the markedly increased oxidative stress in smokers might gradually return to normal but only after a long period of smoking cessation. In conclusion, in the bodies of smokers a series of free radical chain reactions were gravely aggravated, the dynamic balance between oxidation and antioxidation was seriously disrupted, and oxidative stress was clearly exacerbated, which is closely related to many disorders or diseases in smokers. The present study underscored the need, urgency and importance of complete smoking cessation. In order to study effects of cigarette smoking and smoking cessation on plasma constituents and enzyme activities related to oxidative stress, 1255 smokers and 524 healthy nonsmokers were investigated in terms of plasma levels of lipoperoxides (LPO), nitric oxide (NO), vitamin C ( VC, vitamin E (VE) and β carotene (β CAR). Additionally, erythrocytes were examined to deterimine the level of LPO, the activities of superoxide dismutase (SOD), catalase (CAT) and glutathione peroxidase (GSH_Px) showed that, when compared with the average values ​​of the non_smoker group, the average plasma values ​​of LPO, NO and the average erythrocyte value of LPO in the smoker group were significantly increased (P <0.001), while the average plasma values ​​of VC, VE, β CAR, and the average erythrocyte activities of SOD, CAT, GSH_Px were significantly decreased (P <0.001). A linear regression and correlation analysis for 65 male smokers who were all 40 years old showed that with longer Smoking and the daily smoking quantity, the plasma values ​​of LPO, NO and the erythrocyte value of LPO were elevated, while the plasma values ​​of VC, VE, β CAR and erythrocyte values ​​of SOD, CAT, GSH Px were decreased. In a group of 73 smokers who stopped smoking completely for six months, the average plasma values ​​of LPO, NO and the average erythrocyte value of LPO decreased, although they were still significantly higher than those in the matched non_smoker group (P <0.05). Additionally, the average plasma values ​​of VC, VE, β CAR and the average erythrocyte values ​​of SOD, CAT, GSH Px increased, although they were still significantly lower than those in the matched non_smoker group (P <0.05). How ever, after smoking cessation for one year the above average values ​​were not significantly different from those in the matched non_smoker group (P> 0.05). This finding indicates that the markedly increased oxidative stress in smokers might gradually return to no rmalIn conclusion, in the bodies of smokers a series of free radical chain reactions were gravely aggravated, the dynamic balance between oxidation and antioxidation was seriously disrupted, and oxidative stress was clearly exacerbated, which is is related to many disorders or diseases in smokers. The present study underscored the need, urgency and importance of complete smoking cessation.