Transvaginal ultrasound is currently the gold standard for diagnosing polycystic ovaries. The results of studies using ultrasound suggest a prevalence in young women of at least 20%. Between 5% and 10% of these women with polycystic ovaries shown on ultrasound will have the classical symptoms of polycystic ovary syndrome such as infertility, amenorrhoea or signs of hirsutism and obesity, as originally described by Stein and Leventhal in 1935. However, the significance of polycystic ovaries in asymptomatic women is still under investigation, as is the role of Doppler (pulsed and colour) and three-dimensional ultrasound. Ultrasound has also contributed to our understanding of the local and systemic haemodynamic changes associated with polycystic ovaries, although the relationship of these changes to morbidity and mortality is unknown.