Abstract: Objective: To explore the relationship between the expression of cathepsin B (CTSB) and carcinogenesis, infiltration and metastasis of esophageal squamous cell carcinoma. Methods: The expression of CTSB protein and mRNA in 49 cases with tissue from esophageal carcinoma, related paraneoplastic atypical hyperplasia and normal tissue, was determined by immunohistochemistry and in situ hybridization. Results: a) The expression of CTSB protein and mRNA were negative in normal esophageal mucosa. In the group with metastases, the positive rates for the expression of CTSB protein and mRNA in the carcinoma and the atypical hyperplasia tissues were obviously higher than those in the non-metastasis group. There was a significant difference between the two groups (P<0.05). b) The positive expression rates of CTSB protein and mRNA in the cancerous tissues of the group with infiltration of the outer membrane and atypical hyperplasia were both higher than those in the group with infiltration of the superficial and deep muscular layers, and there was a significant difference between them (P<0.05); there was no significant difference between the groups with infiltration of the deep muscular layer and of the superficial muscular layer (P>0.05). c) In the metastasis and non-metastasis group, the positive rates of expression of CTSB protein and mRNA in the cancerous tissues were both higherthanintheatypicalhyperplasia,andtherewasasignificantdifferencebetweenthem (P<0.01).Conclusion: The expression of CTSB protein and mRNA in human esophageal squamous cell carcinoma is rather high, and CTSBis involved in carcinogenesis, infiltration and metastasis ofesophageal carcinoma.