Claudin1 (CLDN1) plays an important role not only in the intercellular barrier function of tight junctions (TJs) but also in migration and invasiveness of cancer cells. Previous reports show that CLDN1 overexpression is significantly related to the malignant behavior in several cancer types whereas its significance in colorectal cancer (CRC) is not fully understood. The present study comprised 119 patients who underwent surgery for CRC, as well as 3 cell lines derived from human CRC. The correlation of gene expression with clinical parameters in patients was assessed by knockdown experiments using 3 cell lines. Patients with high CLDN1 expression were statistically shown to have a relatively better prognosis, and those with low CLDN1 expression showed poorer overall survival and disease-free survival than those with high expression. The assessment of CLDN1 knockdown in the 3 cell lines demonstrated that the siRNA inhibition resulted in a statistically significant increase in cell invasiveness. In conclusion, the present data strongly suggest that CLDN1 expression is a prognostic factor in CRC patients.