Cathepsin B is a matrix protease that may be associated with colorectal carcinoma invasion and progression. In this study, we investigated the localization of cathepsin B in cancerous and noncancerous tissues of 80 patients with colorectal cancer including 25 cases with liver metastasis. In addition, the expression of cystatin C, one of several cathepsin B inhibitors, was compared with that of cathepsin B in the same samples to reveal one of the regulation mechanisms of cathepsin B. The cancer cells in the advancing edge of the tumors often exhibited the strongest immunostaining of cathepsin B, and stromal cells and normal epithelial cells adjacent to the tumors were also positive for cathepsin B. The percentage of cathepsin B-positive cases was significantly larger in the group with liver metastases than in the group without liver metastases. In the group without liver metastases, the cancer cells and stromal cells more frequently exhibited cathepsin B immunoreactivity in Dukes' A cases than in Dukes' B and C cases. In situ hybridization and reverse transcription-polymerase chain reaction (RT-PCR) confirmed cathepsin B synthesis in the cancer and proximal epithelial cells. There was an average 3.7-fold increase in cathepsin B mRNA levels in the cancerous tissues compared with that of noncancerous tissues, and Dukes' A tumors exhibited the highest expression level. Conversely, cystatin C mRNA levels were similar in all samples, and tended to show an inverse correlation with the cathepsin B levels. In conclusion, cathepsin B expression by human colorectal cancers and surrounding noncancerous cell components may contribute to both local invasion at the early stage and remote metastasis without influence of cystatin C.