Borderline ovarian tumors are a low grade form of epithelial ovarian carcinoma with a low rate of growth and a low potential to invade or metastasize. According to the new World Health Organization classification, these tumors are placed between clearly benign and obviously malignant tumors because they exhibit some, but not all, of the morphologic features of malignancy. For a distinction between borderline lesions and cystadenomas or carcinomas, 2 criteria are of the utmost importance: presence of nuclear atypia and absence of stromal invasion. The pathologic subtype ofperitoneal implants is probably one of the main prognosticfactors in patients with serous tumors of low malignant potential, while the prognostic value of micropapillary serous carcinoma in patients with noninvasive implants remains debatable. Although fine needle aspiration (FNA) is the most accurate diagnostic method in cytopathology, its value in the diagnosis of borderline lesions is limited, mainly because of its inability to establish the absence of stromal invasion. The diagnostic accuracy of RNA can be improved by supplementing cytologic examination with such diagnostic techniques as immunocytochemistry and cytometry.