This prospective study was performed with ultrasound (US) to determine the prevalence of unilocular, nonseptated adnexal cysts ("simple cysts") in healthy postmenopausal women and the relationship between cyst occurrence and both hormone replacement and length of time since onset of menopause. Transabdominal and transvaginal US were performed on 149 volunteers aged 50 years or older. Patients were classified according to hormone regimens (no hormones, unopposed estrogen, continuous daily estrogen and progesterone, and sequential estrogen and progesterone) and time since onset of menopause (less than 5 years, 5-10 years, and greater than 10 years). Simple adnexal cysts were found in 22 women with the aid of transvaginal and/or transabdominal US, yielding a relative frequency of 14.8% +/- 5.7% and a prevalence of 14,800 patients with cysts per 100,000 patients. No statistical relationship was found between cyst frequency and type of hormone replacement or length of time since onset of menopause.