Endometrial intraepithelial carcinoma (EIC) is a recently described entity, defined as a noninvasive, cytologically malignant lesion that replaces the endometrial surface epithelium. EIC frequently coexists with uterine serous carcinoma (USC) and is hypothesized to be its precursor lesion. However, the clinical significance and biologic potential of finding EIC without USC is not known. We report three postmenopausal women with EIC alone who were found to have multiple, synchronous foci of extrauterine serous carcinoma at presentation. Because the clinical findings in these patients simulated primary peritoneal serous carcinoma (PSC), we compared the clinicopathologic features of these cases with a group of nine bona fide PSCs for which exhaustively sectioned endometria, fallopian tubes, and ovaries were available for review. The average age of the EIC patients was 73 years. Two patients presented with abdominal distention and one with vaginal bleeding. Hysterectomy in each case showed endometrial polyps with EIC, but without invasive USC, in a background of atrophic endometrium. Bilateral salpingo-oophorectomy and staging showed serous carcinoma involving the ovarian hilum, the surfaces of the fallopian tubes and ovaries, in addition to peritoneal carcinomatosis. p53 overexpression was observed in both EIC and the extrauterine deposits of serous carcinoma in each case. The average age of the PSC patients was 66 years. All nine patients presented with abdominal distention. EIC was not identified in any of the hysterectomy specimens. Bilateral salpingo-oophorectomies, omentectomies, and peritoneal biopsies showed peritoneal carcinomatosis, including bulky peritoneal tumor deposits, but only minimal ovarian surface involvement. p53 overexpression was observed in seven cases. These findings indicate that EIC without coincident USC can be associated with invasive, extrauterine serous carcinomatosis. We did not, however, find any significant differences between the clinicopathologic features of primary extrauterine serous carcinomas (PSCs) and those associated with EIC. We conclude that the finding of EIC in an endometrial curettage specimen should prompt a thorough search for an invasive uterine and/or extrauterine serous carcinoma. Conversely, an endometrial origin should be excluded in patients with peritoneal carcinomatosis.