The slides of all patients with ovarian cystadenocarcinoma treated at the University of Kentucky Medical Center from 1966-1990 were reviewed. Fifty-four serous tumors and 42 mucinous neoplasms were identified for further study. Benign epithelium adjacent to an area of borderline or malignant epithelium was observed in 74 tumors (79%) and a site of epithelial transition was noted in 38 cases (40%). The presence of associated benign epithelium was more common in borderline or well-differentiated lesions and in patients with early-stage disease. These findings are consistent with epidemiologic and molecular genetic data and suggest that certain benign serous or mucinous ovarian tumors have the potential for malignant transformation. Removal of these tumors, particularly in postmenopausal women, should result in a subsequent reduction in the frequency of ovarian cancer.