We investigated the effects of smoking and exposure to environmental tobacco smoke (ETS) on oxidative DNA damage by measuring 8-oxo-7,8-dihydro-2'-deoxyguanosine (8-oxodGuo) levels in DNA of leukocytes of healthy donors (30 smokers, 29 nonsmokers, and 28 ETS-exposed subjects). Nonsmokers had lower 8-oxodGuo levels compared with smokers (5.94 +/- 0.87 x 10(-6) and 19.85 +/- 4.75 x 10(-6) 2-deoxyguanosine, respectively, means +/- SE, P = 0.00007). Subjects exposed to ETS had higher mean value of 8-oxodGuo compared with nonsmokers (9.18 +/- 1.53 x 10(-6) 2-deoxyguanosine, mean +/- SE), nonsignificant by univariate analysis (P = 0.074). Multiregression analysis indicated that the increase of 8-oxodGuo levels induced by ETS was significant (P = 0.045) and that coffee and tea consumption reduced DNA oxidation (P = 0.0053). Oxidative leukocyte DNA damage was positively correlated with plasma cotinine levels in ETS-exposed subjects (r = 0.47, P < 0.01, n = 28) and was increased by age in nonsmokers and ETS-exposed subjects (P = 0.049). The results seem to confirm that ETS exposure is capable of inducing some oxidative DNA damage in circulating leukocytes and that coffee and tea consumption might partially protect against smoking-induced oxidation damage.