In a review of 440 patients treated for endometrial adenocarcinoma at this center since 1974, 21 patients with tumors of papillary histology were identified. Eleven (2.5%) lesions contained histologic changes characteristic of uterine papillary serous carcinoma: complex papillary architecture, high nuclear/cytoplasmic ratio, and irregular epithelial tufting. Ten lesions (2.3%) containing areas of papillary morphology but lacking the criteria for the diagnosis of papillary serous tumors were termed papillary endometrioid adenocarcinoma. Patient age, stage, and the presence of obesity, hypertension, and diabetes were similar in both groups and reflected those characteristics well established for endometrial adenocarcinoma in general. Fewer papillary serous tumors (16.7%) and papillary endometrioid tumors (33.3%) contained progesterone receptors than did other adenocarcinomas (52.3%). In clinical stage I, surgical findings indicating a more advanced stage were present in 40% of patients with papillary serous tumors compared to 10% in papillary endometrioid tumors and 12.5% in nonpapillary adenocarcinomas (P = 0.03, Fisher's exact test). Recurrences were observed in 50% of patients with papillary serous lesions compared to 42.9% in papillary endometrioid lesions and 24.3% in other adenocarcinomas. Survival for clinical stage I papillary serous tumors was worse than that for nonpapillary grade 3 controls (P = 0.042) and survival for papillary endometrioid lesions was not different from that of the same controls. These findings support those of J. L. Chen, D. C. Trost, and E. J. Wilkinson (Int. J. Gynecol. Pathol. 4, 279-288 (1985)) that papillary serous and papillary endometrioid adenocarcinomas represent two distinct subtypes of papillary endometrial neoplasia.