This study compares the clinicopathologic features of 13 pure uterine papillary serous carcinomas (UPSC) with 19 tumors consisting of UPSC admixed with other types of endometrial carcinoma and nine UPSC associated with endometrial polyps. The mean patient age, frequency of preoperative clinical understaging, postoperative pathologic stage, and survival of patients was similar for the three groups. Surprisingly, widespread metastasis, recurrence, and death occurred even in those cases where myometrial invasion amounted to less than 1 mm or where tumor was confined to an endometrial polyp. Poor prognosis appeared to be related to a propensity for vascular invasion and multifocal carcinogenesis. The latter was manifested by the presence of cytologically malignant cells closely resembling the invasive serous carcinoma in the surface endometrium adjacent to the tumor in 89% of cases and in multiple sites in the genital tract and abdomen. This lesion, designated "intraepithelial carcinoma," was present in the endocervix in nine (22%) of the 41 cases, in the fallopian tube in two cases (5%), on the surface of the ovary in four cases (10%), and on peritoneal surfaces or omentum in 10 cases (25%). In addition, we found that UPSC display considerable morphologic heterogeneity. Foci of clear-cell carcinoma were identified in 13 (32%) of the 41 tumors. In five (12%) neoplasms, the invasive component was composed primarily of glands; and in 22 (54%) tumors, thin as opposed to thick papillae predominated. Accordingly, UPSC may be broadly defined as a carcinoma that displays foci of well-differentiated papillae lined by cells that are markedly atypical cytologically. UPSC frequently contain areas of clear cells. Glands with papillary infoldings sometimes predominate in the invasive component. Because the behavior of endometrial neoplasms, in which at least 25% of the carcinoma exhibits a glandular or papillary architecture with serous differentiation, is similar, the term "uterine serous carcinoma" is an appropriate designation for these tumors, regardless of whether other patterns of differentiation are present or whether the tumor is associated with a polyp.