O6-alkylguanine-DNA alkyltransferase (MGMT) represents the first line of defense against the toxic, mutagenic and carcinogenic effects of O 6-alkylguanine adducts in DNA. These adducts mediate the biological activity from a series of alkylating agents, such as the tobacco-specific nitrosamines, believed to contribute to the carcinogenicity of tobacco smoke. There have been conflicting reports on the effects of smoking on MGMT activity in lung and other tissues. Here, we investigate MGMT activity in peripheral blood mononuclear cells (PBMC) and lung bronchial epithelial cells (BEC), extracted by lung brushings, from smokers and nonsmokers attending a bronchoscopy clinic. MGMT activity was significantly lower in BECs (geometric mean; 95% confidence interval 1.02; 0.86-1.20 fmol/μg DNA) than in PBMCs (7.86; 6.70-9.59 fmol/μg DNA; p <0.001), suggesting that bronchial epithelia may be particularly sensitive to alkylation damage. More importantly our results indicate that activity in BECs is significantly decreased in samples from current smokers (0.71; 0.54-0.93 fmol/μg DNA) compared to nonsmokers (1.25; 1.03-1.51 fmol/μg DNA; p = 0.002). This could represent an important contribution to the carcinogenicity of tobacco smoke. © 2006 Wiley-Liss, Inc.