The ovarian surface epithelium (OSE) is the origin of the majority of human ovarian cancers. These adenocarcinomas are characterized by initial local growth followed by spreading into the peritoneal cavity at later stages of tumor progression. The cell-adhesion molecule E-cadherin (E-cad) plays an important role in maintaining tissue integrity. Disappearance or impaired function of E-cad have often been associated with tumor formation and invasion in vivo and in vitro. The cell-specific expression of E-cad was investigated in normal human ovaries (n = 12), in benign (n = 5) and borderline (n = 4) ovarian epithelial tumors and in adenocarcinomas of different stages and histological grades (n = 18), by immunohistochemistry and immunoblotting. An ovarian cancer cell line (NIH-OVCAR3) was used as a reference. The epithelial origin of the cells was confirmed with cytokeratin (AE1/AE3) staining. In normal ovaries, the expression of E-cad was limited to inclusion cysts or deep clefts lined with OSE, whereas no staining of the OSE could be demonstrated at the surface of the ovary. In contrast, benign and borderline tumors uniformly expressed E-cad. This was observed in malignant tumors of all stages despite their degree of differentiation. E-cad was also present in metastasis from such tumors. The cell-specific expression of E-cad in inclusion cysts of normal ovaries and in epithelial layers of borderline tumors indicates a role for E-cad in the early events of the progression to a malignant phenotype. E-cad was not downregulated in later stages of ovarian cancer progression.