Abstract: Objective To explore the expression and clinical significance of claudin-5 in breast cancer. Methods The claudin-5 mRNA and protein expression levels of 88 breast cancer tissues and 61 hyperplasia of mammary tissues were detected by semi-quantitative RT-PCR and Western blotting. The association between claudin-5 levels and clinicopathological characteristics（differentiation degree， lymph node metastasis， age， histological grade， tumor size and clinical stage） was analyzed statistically as well as the prognosis. Results The mRNA and protein level of claudin-5 in the tissues of breast cancer patients were（0.87±0.13） and（0.95±0.23）， higher than（0.23±0.09） and（0.54±0.18） of hyperplasia of mammary tissues with significant difference（P＜0.01）. Claudin5 expression was related with differentiation grade（P＜0.05）， but without age， tumor size and clinical stages（P＞0.05）. The median overall survival was 45.5 months for patients with low expression of claudin-5 protein， higher than 29.5 months for patients with high expression of claudin-5（P＜0.05）. Conclusion There was a relative high expression of claudin-5 in breast cancer. Claudin-5 expression may be associated with breast cancer occurrence and progression， and may serve as a potential indicator for lymphatic metastasis and prognosis in breast cancer.