Polycystic ovaries (PCO) are detected using ultrasonography in a proportion of women who do not have clinical symptoms of the polycystic ovary syndrome (PCOS). The aim of this study was to compare the metabolic and endocrine differences between women with such ultrasound-detected PCO and women with PCOS, and to relate these changes to clinical presentation with particular reference to cycle irregularity. A group of 118 women showing PCO on vaginal ultrasound scan was divided into those who had no hyperandrogenaemia (n = 21) and those who had increased androgens and a clinical presentation normally associated with PCOS (n = 97). These were compared with a reference group of 26 normal subjects. Glucose tolerance, lipid concentrations and endocrine profiles were compared between groups. Apart from higher concentrations of androgens in the PCOS group, there were no significant differences between the PCO and PCOS groups in either fasting and stimulated insulin and glucose or in concentrations of sex hormone-binding globulin, gonadotrophins and blood lipids or in ovarian volume. Both PCO and PCOS subjects with cycle irregularity had significantly higher concentrations of serum fasting and stimulated insulin independent of androgens and body mass index than those with normal cycles. It was concluded that: (i) PCO and PCOS patients have equivalent disturbances in relation to insulin and glucose metabolism as well as lipid and lipoprotein disturbances compared to reference subjects; (ii) higher serum insulin values are associated with menstrual irregularity in both groups; (iii) ultrasound evidence for PCO predicts similar metabolic sequelae to PCOS and can therefore be used for studies of the genetics and long term risks for this condition.