CD24 and CD171 are cell adhesion proteins, which have been shown to be overexpressed in several carcinomas and to be associated with a poor clinical outcome. Our aim was to determine the expression of these two adhesion molecules in ovarian borderline neoplasms. We investigated 50 ovarian borderline tumors (serous, mucinous and endometrioid) as well as 29 benign cystadenomas and 25 carcinomas, which were used as controls. Paraffin sections were stained immunohistochemically for CD24 and CD171, and their expression was recorded in a semi-quantitative manner. In normal epithelium and benign ovarian cystadenomas both the CD24 and CD171 expression was negative to low, while their expression was significantly increased in borderline and malignant ovarian tumors. High-grade carcinomas, and carcinomas with metastases to the omentum presented considerably higher CD24 expression than low-grade carcinomas, and carcinomas without metastases. In addition, a few borderline and many malignant tumors presented cytoplasmic CD24 immunoreactivity, whereas all benign and most borderline tumors showed apical localization of this molecule. In conclusion, borderline tumors and carcinomas of the ovary present increased expression of CD24 and CD171 in relation to their benign counterparts, as is the case in malignant tumors of other organs. Change of staining pattern of CD24 (apical to cytoplasmic) apparently relates to a more aggressive phenotype.