OBJECTIVE To confirm whether there was a familial association between polycystic ovary syndrome (PCOS) and thromboembolic disease, ovarian or breast cancer, diabetes, and heart disease. DESIGN Cross-sectional study. SETTING A university hospital in the United Kingdom. PATIENT(S) Two hundred and seventeen women with and without PCOS under the care of the same consultant gynecologist at a teaching hospital. INTERVENTION(S) Questionnaire survey. MAIN OUTCOME MEASURE(S) Prevalence of a personal or positive family history of thromboembolism, ovarian cancer, breast cancer, diabetes, and heart attacks. RESULT(S) In an analysis of the replies from 41 women with PCOS and 66 controls, we found a statistically significant positive family history of breast cancer and heart attacks among women with PCOS. There was no statistically significant difference in the mean age, ethnic origin, or prevalence of a family history of other diseases. CONCLUSION(S) Our results show a positive association between polycystic ovary syndrome and a family history of breast cancer and heart disease. These associations may be genetic in origin, or secondary to a complex interplay of genetic, intrauterine, and environmental factors. More studies are required to confirm these findings and determine the factors that explain these associations.