Polycystic ovaries (PCO) are highly prevalent in women presenting with hirsutism or recurrent miscarriage but the functional significance of PCO in ovulatory women presenting with infertility remains unclear. We examined the prevalence of PCO, on ultrasonography, among women presenting with infertility. Among 289 couples classified in four main diagnostic categories, PCO were found in 81 (83%) of 98 anovulatory patients, 40 (53%) of 76 patients whose partners had sperm dysfunction, 26 (50%) of 52 patients with tubal disease and in 28 (44%) of 63 patients with unexplained infertility. By comparison, in a control group of 67 parous volunteers, 19 (28%) were found to have PCO. PCO patients with unexplained infertility had higher midfollicular luteinizing hormone and testosterone compared with the group with normal ovaries. The prevalence of PCO was significantly higher in each of the infertility groups than in controls, and a similar tendency (not significant) was observed among women with unexplained infertility. Ovulatory PCO women with infertility had higher testosterone concentrations in comparison with PCO controls. In summary, the prevalence of PCO among ovulatory women with infertility is higher than that in the normal population, suggesting that PCO may, perhaps by virtue of an effect of hyperandrogenaemia, contribute to the causes of subfertility in women with regular menses.