Epithelial differentiation is an early or predisposing step in epithelial ovarian carcinogenesis which occurs in pre-neoplastic lesions, benign tumors and normal ovarian surface epithelium (OSE) of women with a familial history of ovarian cancer. During neoplastic progression, OSE acquires a more epithelial aspect, including the expression of CA125 protein and other epithelial markers, whereas mesenchymal characteristics diminish. In this study, we investigated 26 primary cell cultures, including benign and malignant OSE neoplasms, obtained from women who underwent surgical removal of the ovaries at the university hospital of the State University of Campinas (Campinas, SP, Brazil). Cell morphology was assessed from the time of cell adhesion to the substrate up to the third or fourth passage. CA125 was detected by immunohistochemistry at each passage. Serum CA125 levels were obtained from clinical records and heredograms were constructed using the information about the recurrence of familial cancer provided by the patients. Seventy-eight percent of the malignant OSE tumors analyzed showed an epithelial cell phenotype and 71% percent were positive for CA125. Benign and normal OSE cultures had a fibroblast-like cell phenotype, a negative CA125 expression and an inexpressive history of recurrent familial cancer, compared to malignant OSE tumors. We concluded that the expression of an epithelial phenotype in vitro may serve as an important tumor marker in malignant OSE neoplasms. In certain cases, this marker may be more reliable than the determination of serum CA125 levels. However, the relationship between the expression of the epithelial phenotype in vitro and a familial predisposition to tumor development remains to be determined.