Increased oxidative stress has been observed to contribute the development of insulin resistance. Oxidative stress is known to increase the conversion of deoxyguanosine (dG) to 8-hydroxy-2'-deoxyguanosine (8-OHdG). Human 8-oxoguanine glycosylase (hOGG1) is the key component responsible for the removal of 8-OHdG from oxidatively damaged DNA. The repair activity of the hOGG1 Ser326Cys gene variant has been demonstrated to be lower than that of the hOGG1 Ser/Ser genotype. Therefore, the possible association of the hOGG1 Ser326Cys gene variant with insulin sensitivity was investigated in 279 normal glucose-tolerant subjects without history of cancer. Allele frequency was 21.5% for the Ser/Ser genotype (n = 60), 45.9% for the Ser/Cys genotype (n = 128), and 32.6% for the Cys/Cys genotype (n = 91). Subjects carrying the Cys/Cys genotype had significantly lower insulin sensitivity levels, assessed by homeostasis model assessment-insulin resistance (HOMA-IR), compared with the Ser/Ser and Ser/Cys genotypes (P < 0.001 and P < 0.001, respectively). In a multiple linear regression analysis, the Cys/Cys genotype was a significant determinant of HOMA-IR, independent of age, sex, body mass index, fasting plasma cholesterol, triglyceride, HDL cholesterol, LDL cholesterol, or hypertension. The present study indicates that the hOGG1 gene Cys/Cys variant is associated with a significant decrease in insulin sensitivity in subjects with normal glucose tolerance.