Tobacco smoke contains many carcinogens which can lead to DNA methylation. Glutathione-S-transferases (GSTs) are detoxifying enzymes, and the effects of the genes GSTM1, GSTT1, and GSTP1 on cigarette smoke-induced methylated adducts have not been elucidated. Therefore, we investigated the association of the GSTM1, GSTT1, and GSTP1 genes and N7-methylguanine (N7-MeG) adducts in smokers. Urinary N7-MeG concentrations were measured by using liquid chromatography-tandem mass spectrometry in 112 smokers and 89 non-smokers, and GSTM1, GSTT1, and GSTP1 genotypes were identified by polymerase chain reaction. Smokers had higher N7-MeG concentrations than did non-smokers (3238+/-305 ng/mg creatinine [standard error] vs. 2386+/-153 ng/mg creatinine; P=0.01). Higher N7-MeG concentrations were observed with the GSTM1 null genotype than with the GSTM1 non-null genotype (3230+/-292 ng/mg creatinine vs. 2336+/-153 ng/mg creatinine; P=0.007), particularly in smokers (3775+/-483 ng/mg creatinine vs. 2468+/-228 ng/mg creatinine; P=0.02). However, the GSTT1 and GSTP1 genotypes were not associated with urinary N7-MeG concentrations. Therefore, the susceptible GSTM1 genotype may modulate the concentrations of N7-MeG adducts in the DNA of smokers.