The occurrence of mural nodules in cystic common epithelial tumors of the ovary is well established, but differences in terminology and difficulties in histopathologic interpretation have hampered adequate understanding of their differential diagnoses and prognoses. Using immunohistochemistry and ultrastructural analyses supplementally, we studied two cases, one a serous cystadenocarcinoma with nodules of undifferentiated sarcoma, the other a mucinous cystadenocarcinoma with mural nodules of anaplastic carcinoma. Based on these cases and a review of 48 cases in the literature, we propose a standardized terminology. Mural nodules may be either reactive or neoplastic. Neoplastic mural nodules may be composed of benign elements, carcinoma, carcinoma with reactive elements, sarcoma, or an admixture of carcinoma and sarcoma; the latter category has some similarities to malignant mixed mesodermal tumors. The prognosis of patients with malignant mural nodules is poor, with 50% mortality. Strict morphologic criteria supplemented by immunohistochemistry aids in the sometimes difficult differential diagnosis among these types of mural nodules.