Specific and recurrent chromosome abnormalities may occur in regions of the genome that are involved in the conversion of normal cells to those with tumorigenic potential. Ovarian cancer is the primary cause of death among patients with gynecologic malignancies. We performed cytogenetic analysis in a subgroup of epithelial ovarian tumors, the endometrioid tumors, which are histologically indistinguishable from endometrial carcinoma of the uterus. We studied 10 endometrioid tumors to determine the degree of cytogenetic similarity between these two carcinomas. Six of 10 endometrioid tumors showed a near-triploid modal number, and one had a tetraploid modal number. Eight of the 10 contained structural chromosome abnormalities, of which the most frequent were 1p-- (5 tumors), 6q-- (4 tumors), 19q+ (4 tumors), and chromosome 3 rearrangements (4 tumors). These cytogenetic results resemble those reported for papillary ovarian tumors and differ from those of endometrial carcinoma of the uterus. We conclude that despite the histologic similarities between the endometrioid and endometrial carcinomas, the genetic abnormalities in the genesis of these tumors differ significantly.