Primary squamous cell carcinoma of the ovary is extremely rare. We studied a 58-year-old woman in whom a keratinizing squamous cell carcinoma of the ovary had arisen from a mucinous cystic tumor of endocervical (müllerian) type. The tumor was interpreted initially as a transitional cell carcinoma of the ovary with marked squamous differentiation, but there was no evidence of either transitional cell carcinoma or malignant Brenner tumor. Furthermore, features of dermoid cyst (mature cystic teratoma), endometriosis, or adenosquamous carcinoma were not seen. The mucinous columnar epithelial component was largely benign and only focally proliferative or borderline. As found typically in endocervical (müllerian) mucinous tumors, numerous polymorphonuclear leukocytes were seen in the stroma and the neoplastic mucinous epithelium.