Twenty-one of 34 simultaneous cancers involving the endometrium and ovary were classified as endometrial primary tumors with ovarian metastases. The criteria for this classification were either a multinodular ovarian pattern (major criterion) or two or more of the following minor criteria: small (less than 5 cm) ovary(ies), bilateral ovarian involvement, deep myometrial invasion, vascular invasion, and tubal lumen involvement. Twelve cancers were classified as independent neoplasms, primarily by the absence of the above criteria. Although they were classified as independent, the histologic features of the endometrial and ovarian tumors were the same in 11 of these 12 cases. Only one case represented an ovarian primary tumor with an endometrial metastasis. Both the group believed to have endometrial primaries with ovarian metastases and that with independent primaries showed high incidences of associated endometrial hyperplasia, supporting the belief that the endometrium is a primary site in both groups. The cancers classified as metastatic, with no known spread outside the endometrium-myometrium and ovary, were found to involve other sites significantly (P less than 0.01) more frequently than those classified as independent. Grade 3 endometrioid carcinoma, adenosquamous carcinoma, and malignant mixed müllerian tumors occurred only in the metastatic group, whereas the independent group had a variety of endometrioid and nonendometrioid tumors.