BACKGROUND: To study the prevalence of polycystic ovaries (PCO) in women of reproductive age. METHODS: A total of 189 healthy volunteers aged 20-45 years were examined. The subjects were divided into two groups according to age: or =36 years. Transvaginal ultrasonography was performed and blood samples were collected on cycle day 1-6. RESULTS: The prevalence of PCO in the entire study population was 14.2% (27/189). In the age group of or =36 years 7.8% (8/101). Compared to women with normal ovaries, those with PCO had significantly higher serum testosterone (T) concentrations. Women with PCO tended to have lower serum FSH concentrations and higher LH/FSH ratios than controls. Women with PCO had significantly more irregular cycles (44% vs. 19%, p=0.001) and problems in conceiving (25.9% vs. 9.2%, p=0.01) than women with normal ovaries. CONCLUSIONS: The findings demonstrate that the prevalence of PCO in healthy women varies with age, being more common among women aged or =36 years. Although the hormonal parameters and clinical findings among women with PCO mimicked those of PCOS, it remains unclear if these women will later develop full-blown syndrome.