Sialyl-Tn (sTn) is a tumor-associated carbohydrate antigen overexpressed in many carcinomas. But whether it is expressed in neoplastic human endometrium is controversial. Here we screened 111 clinical samples including 82 endometrial cancers, 16 atypical hyperplastic and 13 normal endometria, by immunohistochemistry using a novel anti-sTn antibody, namely 3P9. Our results showed that the expression of sTn in the endometrial carcinoma was significantly higher (76%) than that in normal tissues (31%) (P < 0.01) and in atypical hyperplasia (44%) (P < 0.05). Importantly, we found that sTn expression in typeⅠ carcinoma (80%) was obviously higher than that in typeⅡ(45%) (P < 0.05). These results suggested that sTn may be a biomarker for endometrial carcinoma, especially for typeⅠcarcinoma. Moreover, we observed that sTn expression was increased with the decline of tumor histological grade (P < 0.05), indicating that sTn was associated with a favorable prognosis in endometrial carcinoma.