Survivin is a new member of the inhibitor of apoptosis family of anti-apoptotic proteins. It has been reported that survivin is expressed during fetal development and in cancer tissues. Because suppression of apoptosis is important for carcinogenesis and tumor growth, we investigated the expression of survivin in human endometrial carcinomas. We analyzed serial frozen sections for survivin protein expression in 31 cases of endometrial carcinoma and 20 cases of normal endometria by fluorescent immunohistochemistry. We analyzed the relationship between the percentages of survivin-stained cells and the patient's characteristics, including clinical stage, histological grade, presence of invasion to >1/2 myometrium, clinical outcome, and survival rate. Survivin was weakly detected in some normal endometria in the proliferative phase (0-5.1%) and in the secretory phase (0-15.8%). There was, however, abundant survivin immunoreactivity in the nucleus and/or cytoplasm of the endometrial carcinoma cells. Scoring on the basis of the percentage of positive cells indicated that survivin expression was significantly associated with proliferating cell nuclear antigen-labeling index, clinical stage, histological grade, the presence of invasion to >1/2 myometrium, clinical outcome, and survival rate (P<0.01, respectively). We conclude that the survivin protein is a defining diagnostic marker for endometrial carcinomas that may also yield prognostic information.