To assess the level of circulating endothelial progenitor cells (CEPC) in cycling women compared with men and menopausal women.Controlled clinical study.Healthy, nonsmoking volunteers.Twelve women, aged 18–40 years, with regular menstrual cycles, 12 menopausal women, and two groups of 12 age-matched men were recruited. Women did not receive any hormone therapy.Collection of 20 mL of peripheral blood.The number of CEPC, defined as (Lin−/7AAD−/CD34+/CD133+/KDR+) cells per 10 6 mononuclear cells (MNC), was measured by flow cytometry.The number of CEPC was significantly higher in cycling women than in age-matched men and menopausal women (26.5 per 10 6 MNC vs. 10.5 per 10 6 MNC vs. 10 per 10 6 MNC, respectively). The number of CEPC was similar in menopausal women, age-matched, and young men.The number of CEPC is influenced by an age–gender interaction. This phenomenon may explain in part the better vascular repair and relative cardiovascular protection in younger women as compared with age-matched men.