CONTEXT: Polycystic ovary syndrome (PCOS) is associated with menstrual and reproductive abnormalities, insulin resistance, and obesity. OBJECTIVE: The objective of this study was to determine the prevalence of diagnosed PCOS and its association with cardiovascular risk factors. SETTING: The study is set in an integrated health care delivery system in northern California. PATIENTS: A total of 11,035 women with PCOS were identified by one or more outpatient diagnoses of PCOS using health plan databases. An age-matched sample of women without PCOS was also selected. OUTCOME MEASURES: Prevalence of PCOS and targeted cardiovascular risk factors [hypertension, dyslipidemia, diabetes mellitus, and body mass index (BMI)] were measured. RESULTS: During 2002-2004, the prevalence of diagnosed PCOS among female members aged 25-34 yr was 2.6% (95% confidence interval 1.6-1.7%). Women with diagnosed PCOS were more likely than those without PCOS to be obese [BMI > or = 30 mg/m(2); odds ratio (OR) 4.21, 3.96-4.47]. Furthermore, PCOS was associated with diabetes (OR 2.45, confidence interval 2.16-2.79), hypertension (OR 1.41, 1.31-1.51) and known dyslipidemia (OR 1.53, 1.39-1.68), even after adjusting for BMI and known confounders. Among women with PCOS, compared with whites, Blacks and Hispanics were more likely and Asians less likely to be obese; Asians and Hispanics were more likely to have diabetes; and Blacks were more likely and Hispanics less likely to have hypertension. CONCLUSIONS: Within a large, community-based population receiving health care, diagnosed PCOS was highly prevalent and associated with a much higher frequency of cardiovascular risk factors that varied by race/ethnicity. Our prevalence estimates likely underestimate the true prevalence of PCOS. Further studies are needed to explore racial/ethnic differences and the extent to which PCOS contributes to future cardiovascular risk.