A case of bilateral ovarian endometrial stromal sarcoma and peritoneal implants associated with ovarian and uterine endometriosis in a 48 year old woman is presented. Focal differentiation into areas of sex-cord stromal tumour were found. Immunohistochemistry for both the ovarian tumour and the sex-cord stromal tumour was carried out. The morphology and differential diagnosis are discussed together with a review of the literature.