Mesonephric (wolffian) neoplasms of the female genital tract are infrequent and found in sites where embryonic remnants of wolffian origin are usually detected, such as the uterine cervix, broad ligament, mesosalpinx, and ovary. Their diagnosis is difficult because of the absence of specific immunohistochemical markers for mesonephric derivatives. We present the first report of adenocarcinoma of mesonephric type arising as a purely myometrial mass without endometrial or cervical involvement in the uterine corpus of a 33-year-old woman. The tumor showed a combination of patterns, with retiform areas, ductal foci, and small tubules with eosinophilic secretion, which merged with solid sheets of cells with a sarcomatoid appearance. Immunohistochemically, neoplastic cells were diffusely positive for cytokeratin 7, epithelial membrane antigen, and CD15 and focally positive for BerEP4 and vimentin. A hitherto unreported feature was the positivity for CD10 in neoplastic cells, which was also present in a large number of control tissues obtained from male mesonephric derivatives and female mesonephric remnants and tumors. Furthermore, CD10 was negative in controls from müllerian epithelia of the female genital tract and in their corresponding tumors. Therefore, the expression of CD10 by mesonephric remnants may be useful in establishing the diagnosis of tumors with mesonephric differentiation.