Synchronous ovarian primaries are infrequently found in patients with endometrial cancer. Although numerous investigators have examined the characteristics of these women, most include patients with tumors of similar histology, which may simply represent ovarian metastases. To overcome this problem, we present here patients found to have tumors of dissimilar histology. Of 499 patients with endometrial cancer undergoing primary surgery between 1980 and 1997, 18 (3.6%) were found to have endometrial and ovarian primaries of dissimilar histology. The median age was 64.2 years. Most had stage I, grades I and II, minimally invasive endometrial adenocarcinomas and stage IA mucinous or serous ovarian cystadenocarcinomas. Most ovarian tumors were either borderline or grades I and II. The 5-year actuarial disease-free (DFS) and cause-specific survivals of the entire group were 81.2% and 89.5%, respectively. Those with both stage I ovarian and endometrial primaries had a trend to a better DFS (100 versus 68.6%,p= 0.07) than did women with higher stage disease. Our data demonstrate that synchronous ovarian primaries of dissimilar histology are infrequently found in women undergoing surgery for endometrial cancer. These women seek treatment at a relatively advanced age, and have early-stage, low grade disease in both sites. Their outcome is favorable, particularly those with stage I disease in both sites.