We measured 8-hydroxy-2-deoxyguanosine (8-OHdG) levels in human leukocytes from healthy donors to evaluate oxidative DNA damage and its correlation with smoking, physical exercise, and alcohol consumption. A significant increase in oxidative DNA damage was induced by cigarette smoke, with the mean level of 8-OHdG being significantly higher in smokers (33.1 ± 10.6 per 10⁶ 2-deoxyguanosine (dG) [mean ± SE], n = 16) compared with nonsmokers (15.3 ± 1.8 per 10⁶ dG, n = 31) and former smokers (17.8 ± 1.5 per 10⁶ dG, n = 9). The highest values were observed after smoking more than 10 cigarettes per day (41.8 ± 17.1 per 10⁶ dG, n = 9). A large interindividual variation in 8-OHdG levels was observed in all analyzed groups. We also observed a correlation between 8-OHdG levels and age in nonsmokers and former smokers. Neither frequency of physical exercise nor alcohol drinking significantly modified 8-OHdG levels in leukocytes.