Both onset and cessation of menstruation have strong genetic inclination. We aimed to identify genetic factors influencing the onset of menarche and natural menopause in a Japanese population by investigating the polymorphisms of estrogen receptor-alpha and estrogen-metabolizing enzyme genes. Three hundred seventeen postmenopausal Japanese women, aged 46 yr and over, were enrolled in this study under informed consent. Genomic DNA was extracted from peripheral leukocytes, and PCR-based restriction fragment length polymorphism assays were used to determine estrogen receptor-alpha: PvuII, XbaI, and estrogen-metabolizing enzymes; CYP17, estrogen biosynthesis (high activity, A2/A2, CYP1A1), hydroxylation (high inducibility, vt/vt, and COMT), inactivation (low activity, L/L) genotypes. There were no significant differences in ages at menarche and natural menopause or years of menstruation among each PvuII or XbaI genotype and seven combinations of PvuII and XbaI genotypes. We found that ages at menarche in women with A1/A2 (higher activity of CYP17; 13.6 +/- 1.2 yr) were significantly earlier than in those with A1/A1 (lower activity of CYP17; 14.1 +/- 1.3 yr). There were no significant differences in age at natural menopause and years of menstruation among each CYP17, CYP1A1, or COMT genotype. The small sample size of each combination of estrogen-metabolizing genotypes made it impractical to evaluate the effects of the interdependency of each genotype, including extreme genotype categories such as A2/A2L/Lvt/vt vs. A1/A1H/Hwt/wt genotypes, on ages at menarche and/or natural menopause. The results suggest that the estrogen-metabolizing CYP17 genotype influences age at menarche in healthy postmenopausal Japanese women.