Although in endometrioid type endometrial carcinoma depth of invasion is a powerful predictor of extrauterine disease and survival, in serous carcinoma its importance is unclear. Recurrences and death in patients with serous tumors confined to the endometrium or an endometrial polyp have been reported. In other studies, however, the absence of myometrial invasion was correlated with a more favorable course. In an attempt to clarify this issue, we reviewed 13 completely staged, stage IA serous carcinomas with follow-up from 10 to 93 months (median 38), in which extensive histologic examination had been performed. Serous carcinoma was identified in an endometrial polyp in six cases, in an endometrial polyp and associated endometrium in four, and solely in the endometrium in three cases. No other histologic types of endometrial carcinoma were present, and there was no myometrial invasion. Multifocal serous intraepithelial carcinoma was also seen in 12 cases. Two of the patients died of disease with intraabdominal carcinomatosis at 10 and 14 months after presentation. The overall estimated survival was 83%, showing a relatively favorable prognosis. In conclusion, although the absence of histologically detected myometrial invasion may be associated with recurrences and death in serous carcinoma, an accurately assessed stage based on a careful histologic examination appears to be, at present, the most reliable predictor of survival.