BACKGROUND Women with polycystic ovary syndrome (PCOS) are assumed to be at increased risk of endometrial cancer (EC), albeit of a more differentiated type with better prognosis than in normal women. This study was designed to test these assumptions, as evidence for them is lacking. METHODS The prevalence of polycystic ovaries (PCO), as a marker of PCOS, was investigated in ovarian sections from 128 women with EC and 83 with benign gynaecological conditions. The expression of the prognostic markers p53, Ki67, Bcl2 and cyclin D1 was also investigated by immunohistochemistry in endometrial tumours from 11 women with PCO and 16 with normal ovaries. RESULTS Overall, PCO were similarly prevalent in women with EC (8.6%) and benign controls (8.4%); however, in women aged <50 years, PCO were more prevalent in women with EC (62.5 versus 27.3%, P = 0.033). Cyclin D1-expressing endometrial tumours tended to be more prevalent in women with PCO compared to normal ovaries (36.4 versus 6.25%, respectively, P = 0.071). Bcl2-, p53- and Ki67-expressing tumours were similarly prevalent. CONCLUSIONS The association between PCOS and EC appears confined to premenopausal women. The tendency for cyclin D1-expressing endometrial tumours to be more prevalent in women with PCO challenges the assumption that EC prognosis is improved in women with PCOS.