Metabolic disturbances associated with insulin resistance are present in most women with polycystic ovary syndrome. This has led to suggestions that women with polycystic ovary syndrome may be at increased risk of cardiovascular disease in later life. We undertook a long-term follow-up study to test whether cardiovascular mortality is increased in these women. A total of 786 women diagnosed with polycystic ovary syndrome in the United Kingdom between 1930 and 1979 were traced from hospital records and followed for an average of 30 years. Standardized mortality ratios (SMRs) were calculated to compare the death rates of these women with national rates. The SMR for all causes was 0.90 (95% CI, 0.69-1.17), based on 59 deaths. There were 15 deaths from circulatory disease, yielding an SMR of 0.83 (95% CI, 0.46-1.37). Of these 15 deaths, 13 were from ischemic heart disease (SMR 1.40; 95% CI, 0.75-2.40) and two were from other circulatory disease (SMR 0.23; 95% CI, 0.03-0.85). There were six deaths from diabetes mellitus as underlying or contributory cause, compared with 1.7 expected (odds ratio 3.6; 95% CI, 1.5-8.4). Breast cancer was the commonest cause of death (SMR 1.48 based on 13 deaths; 95% CI, 0.79-2.54). We conclude that women with polycystic ovary syndrome do not have markedly higher than average mortality from circulatory disease, even though the condition is strongly associated with diabetes, lipid abnormalities, and other cardiovascular risk factors. The characteristic endocrine profile of women with polycystic ovary syndrome may protect against circulatory disease in this condition.