Aberrant activation of the Wnt signaling pathway has been implicated in tumorigenesis of a wide range of tumors, including colorectal cancer. Regarding endometrial stromal tumors and related high-grade sarcomas, there have been some reports regarding nuclear accumulation of beta-catenin. To clarify the function of the aberrant Wnt signaling pathway in these tumors, we searched for mutations of the CTNNB1 (beta-catenin) gene and APC gene by PCR direct sequencing and analyzed the methylation status of SFRP genes. We also examined overexpression of cyclin D1 and MMP-7, which are direct target genes of beta-catenin. Eight endometrial stromal nodules, 16 low-grade endometrial stromal sarcomas, and 13 undifferentiated endometrial sarcomas were examined. PCR and direct sequencing revealed no mutation of the beta-catenin gene or the APC gene. Concerning the promoter methylation status of SFRP genes, methylation-specific PCR revealed no significant difference between the group with nuclear beta-catenin expression and that without nuclear beta-catenin expression. Immunohistochemistry revealed overexpression of cyclin D1 in 2 out of 8 endometrial stromal nodules, 1 out of 17 low-grade endometrial stromal sarcomas, and 6 out of 13 undifferentiated endometrial sarcomas, and these 6 undifferentiated endometrial sarcomas simultaneously expressed nuclear beta-catenin. Interestingly, all six undifferentiated endometrial sarcoma cases with cyclin D1 overexpression histologically featured rather uniform nuclei. In contrast, the six cases of undifferentiated endometrial sarcoma with highly pleomorphic nuclei were all negative for cyclin D1. In conclusion, among endometrial stromal tumors and related sarcomas, undifferentiated endometrial sarcomas featuring uniform nuclei were characterized by frequent coincident expression of beta-catenin and cyclin D1. This finding raises the possibility that cyclin D1 is upregulated by beta-catenin in these high-grade sarcomas previously called high-grade endometrial stromal sarcoma.