Epithelioid sarcoma is a rare but distinctive soft tissue tumor showing characteristic histologic features associated with a typical course, but there is still some debate as to its histogenesis. Two cases of this unique tumor were analyzed using the electron microscope. Light microscopically the diagnostically relevant cell type is oval to polyhedral exhibiting a dense eosinophilic cytoplasm. In such epithelioid cells the cytoplasm may sometimes be completely filled with an inclusion-body like hyaline eosinophilic material. Ultrastructurally the most striking feature are abundant intermediate-type filaments in these eosinophilic cells. While none of the electron microscopic findings is specific, the accumulation of the intermediate-type filaments in conjunction with the other cellular features described in sufficiently consistent and distinctive and can thus be used for confirming the diagnosis. In formalin-fixed specimens the electron microscopical investigation may be helpful in everyday diagnosis. The suggestion from the literature that epithelioid sarcoma is related to the synovial sarcoma can not be supported by our ultrastructural findings.