The prevalence of polycystic ovaries (PCO) in normal women of reproductive age was determined by pelvic ultrasound scanning of 257 volunteers who considered themselves to be normal and who had not sought treatment for menstrual disturbances, infertility, or hirsutism. All women had completed a menstrual history questionnaire. 99 women were on oral contraceptives at the time of the study. Of the 158 subjects who were not on oral contraceptives 18% had irregular cycles. 116 (73%) women had normal ovaries and 36 (23%) had PCO. 5 women had multifollicular ovaries and 1 had small, unstimulated ovaries. Only 1 woman with normal ovaries had an irregular menstrual cycle. Of the women with PCO, 76% had irregular cycles, and 6 of the 8 with regular cycles were hirsute. Women with and those without PCO differed in distribution of serum LH concentrations although the median values were similar. 25% of women with PCO had LH concentrations which exceeded the upper limit of the normal range. Thus PCO are common in normal women. Some of these women may have clinical and biochemical markers of PCO, which suggest that PCO in women who consider themselves to be normal is part of the same clinical spectrum as the classic Stein-Leventhal syndrome.