We report an undifferentiated sweat gland carcinoma of the vulva in an 80-year-old woman. The tumor, which was located in the right labium majus, resembled an epithelioid sarcoma histologically; it had a granulomatous appearance with multiple tumor nodules containing epithelioid tumor cells. The tumor also contained rhabdoid cells; a large cluster of them showed histological features indistinguishable from those of a malignant rhabdoid tumor. Immunohistochemically, the tumor cells reacted not only for epithelial markers such as cytokeratins, EMA, and CEA, which are known to be expressed by epithelioid sarcoma, but also for CA125 and with monoclonal antibodies recognizing sweat gland structures--namely, EKH5 and EKH6. For comparison, two epithelioid sarcomas and two extrarenal malignant rhabdoid tumors were also studied. Of these tumors, only one extrarenal rhabdoid tumor reacted with EKH5, and none reacted for CA125. Electron-microscopic examination of the present tumor showed the presence of discontinuous basal laminae and tonofibril-like structures as well as primitive cell junctions and interdigitating filopodia. From these findings, we conclude that the tumor was an undifferentiated sweat gland carcinoma mimicking an epithelioid sarcoma. Findings in this case support the idea of the diverse histogenesis of extrarenal malignant rhabdoid tumors and indicate that electron microscopy is important for differentiating epithelioid sarcoma from skin adnexal carcinoma.