We describe an ovarian mucinous cystadenocarcinoma with several sarcoma-like mural nodules (SLMN). The distinction between these lesions and foci of anaplastic carcinoma is important because of the poor prognosis of the latter. We have studied the potential value of immunohistochemistry in the differential diagnosis of these two lesions. In contrast to an anaplastic carcinoma, which was largely composed of keratin-positive cells, SLMN were negative or only focally positive. Therefore, in distinguishing SLMN from foci of anaplastic carcinoma, keratin strains may be added to other gross and microscopical differential features, such as size, demarcation, and presence or lack of obvious carcinomatous elements.