Most mucinous carcinomas of the ovary (80% of cases) show a continuum from benign to borderline to frankly malignant areas that are often found in close proximity to each other. About 20% of cases are composed entirely of malignant areas. In contrast to the borderline mucinous tumors, mucinous carcinomas are not subdivided into intestinal and endocervical types. The benign areas (right side of the cyst wall) consist of a single layer of intestinal-type tall columnar mucinous cells with basally located hyperchromatic nuclei. The borderline areas (left side of the cyst wall) show multilayering and tufting of epithelium as well as branching papillary structures. The papillae are either stroma-free or have thin fibrous cores. There is mild to moderate cytologic atypia consisting of nuclear enlargement and hyperchromasia with occasional prominent nucleoli. The appearance resembles a hyperplastic or adenomatous colonic polyp.