Endometrioid tumors are part of the surface epithelial tumor group of ovarian neoplasms (10-20% of which are the endometrioid type). Benign and borderline variants are rare, as the majority are malignant. There is an association with endometriosis and concurrent primary endometrial carcinoma (endometrial cancer).

On gross pathological examination, the tumour is cystic and may be solid and some arise in cystic endometriosis. In 17% of cases, endometrioid tumours are found bilaterally. Microscopically, there is a resemblance to endometrial carcinoma, and over a third of cases have focal squamous differentiation.