A case of a pure heterologous sarcoma of the uterine corpus composed exclusively of rhabdomyosarcomatous elements has been studied by multiple morphologic and biochemical techniques. The neoplasm filled the endometrial cavity and protruded out the cervical os, but the myometrium was only superficially invaded. The tumor did not extend outside the corpus. The pathologic features are discussed in detail. Evidence of striated muscle differentiation could be identified on light microscopic and ultrastructural examination. Immunoperoxidase staining of tumor cells with antibodies to myoglobin were positive. Histochemical preparations for lactate dehydrogenase, succinic dehydrogenase, and acid phosphatase were also positive in neoplastic cells. Other stains gave equivocal or negative results. These findings are discussed in comparison with previous reports. © 1983 International Society of Gynecological Pathologists.