BACKGROUND: Polycystic ovary syndrome (PCOS) is associated with insulin resistance and features in common with the metabolic syndrome (MetS)--factors shown to predict cardiovascular risk and type 2 diabetes. We investigated the prevalence and characteristics of the MetS in PCOS by three definitions-World Health Organization (WHO), National Cholesterol Education Program Adult Treatment Panel III (NCEP-ATP-III) and International Diabetes Federation (IDF)--and compared that with the background population. METHODS: Cross-sectional study of 168 women with PCOS and 883 age-matched controls from the Australian Diabetes, Obesity and Lifestyle (AusDiab) study. RESULTS: Prevalence of the MetS in PCOS subjects was 33% by WHO, 37% by NCEP-ATP-III and 40% by IDF criteria, compared with 10% by NCEP-ATP-III and 13% by IDF in controls (P < 0.001). MetS by WHO criteria was not calculated in the AusDiab population. Age was an independent predictor of MetS in PCOS and controls. The prevalence of MetS was significantly higher among those with PCOS (P = 0.027) in obese women (BMI > 30 kg/m(2)), and higher but not significantly so in overweight (BMI 25-30 kg/m(2)) women (P = 0.052). Dehydroepiandrosterone sulphate was associated with a lower risk of the MetS--Odds ratio 0.86 (95% confidence interval, 0.77-0.97, P = 0.011). CONCLUSIONS: An approximate 4-fold increase in the prevalence of the MetS in women with PCOS compared with the general population, consistent with the proposed major role of insulin and obesity in the syndrome, implies greater risk of cardiometabolic disease in women with PCOS. However, this estimate is likely to vary according to PCOS definition, ethnicity and different aetiological pathways to PCOS.