According to the Rotterdam 2003 criteria, an ovary is defined as polycystic if 12 or more follicles of 2-9 mm are present, when evaluating the ovary by ultrasonography on Days 3-5 of the menstrual cycle in women not using hormonal contraceptives. The aim of this population-based study was to estimate the prevalence of polycystic ovaries (PCO) in a representative sample of young Danish women according to the Rotterdam criteria.