Oxidatively damaged DNA base lesions are considered to be mainly repaired by 8-oxoguanine DNA glycosylase (OGG1) mediated pathways. We investigated the effect of the OGG1 Ser326Cys polymorphism on the level and repair of oxidatively damaged DNA in mononuclear blood cells (MNBC) by means of the comet assay. We collected blood samples from 1,019 healthy subjects and genotyped for the OGG1 Ser326Cys polymorphism. We found 49 subjects homozygous for the variant genotype (Cys/Cys) and selected same numbers of age-matched subjects with the heterozygous (Ser/Cys) and homozygous wild-type genotype (Ser/Ser). Carriers of the Cys/Cys genotype had higher levels of formamidopyrimidine DNA glycosylase (FPG) sensitive sites in MNBC (0.31 ± 0.03 lesions/10(6)bp) compared to Ser/Ser (0.19 ± 0.02 lesions/10(6)bp, P