The polycystic ovary syndrome (PCOS) is the most common endocrine disturbance affecting women, but disagreements in diagnostic criteria make it difficult to compare epidemiological studies on long-term health risks such as cancer. The association between PCOS and endometrial adenocarcinoma has been reported for many years. Although the degree of risk has not been clearly defined, it is generally accepted that for women with PCOS who experience symptoms of amenorrhoea or oligomenorrhoea, the induction of artificial withdrawal bleeds to prevent endometrial hyperplasia is prudent management. Studies examining the relationship between PCOS and breast carcinoma have not always identified a significantly increased risk, although one recent study examined the standardized mortality rate (SMR) calculated for patients with PCOS compared with the normal population and found that the SMR for all neoplasms was 0.91 (95% CI 0.60-1.32) and for breast cancer 1.48 (95% CI 0.79-2.54). Few studies have addressed the possibility of an association between polycystic ovaries and ovarian cancer, and the results are conflicting and generally reassuring.