In a review of 518 nonirradiated hysterectomy specimens with endometrial carcinoma and 39 with carcinosarcoma, 86 cases were identified in which there were microscopic foci of malignant epithelium that failed to alter the architecture of an otherwise thin atrophic or weakly proliferative endometrium or endometrial polyp. These changes were interpreted to be endometrial carcinoma in situ (ECIS). This change was present on the surface (SCIS), in isolated glands (GCIS), and in small (three to five) groups of glands (thin AH). For these three subtypes, neither papillations, stromal reaction, nor glandular epithelial bridging were present. When they were present and there was either no stromal reaction or the reaction was limited to one or two glands in the absence of myometrial invasion or endometrial architectural alteration, they were designated as thin carcinoma (thin CA), a fourth subtype of ECIS. For these patients the age range was 50-90 years, with mean and median ages of 66 and 65 years, respectively. All but one patient was known to be postmenopausal. In 66% of cases the endometrial malignancy either was uterine papillary serous/clear cell carcinoma (UPS) or contained an element of UPS. Foci of ECIS were present adjacent to the tumor in 86%, isolated from the tumor in 59%, and diffuse in 16% of cases. It is proposed that in many postmenopausal women, either surface epithelium or individual glands undergo malignant transformation and then progress to an invasive malignancy without an intervening phase of atypical hyperplasia, particularly in cases with an element of UPS.