Claudin proteins represent a large family of integral membrane proteins crucial for tight junction (TJ) formation and function and are abnormally regulated in several human cancers. The aim of the present study was to study the expression levels of claudin-5 in pre-malignant disease as borderline mucinous ovarian tumors. Previous reports have suggested that claudin-5 over-expression correlates with aggressive behaviour in serous ovarian adenocarcinoma, breast cancer and in pancreatic andenocarcinoma.We investigated the expression of claudin-5 in mucinous ovarian borderline tumors and its correlation with clinico-pathological parameters and the expression of serum markers cancer antigen (CA) 125 and tumor-associated trypsin inhibitor (TATI).A total of 29 mucinous borderline tumor tissue samples were analyzed using immunohistochemical staining for claudin-5. An association between strong claudin-5 expression and higher serum levels of TATI (p=0.04) and CA125 (p=0.008) were found. There was also an association between claudin-5 expression and the presence of ascites (p=0.02).Changes in claudin-5 expression may play a role in malignant transformation.