Tumor cell invasion and metastasis are associated with the proteolytic activity of various types of proteinases. Among them, cathepsins, which are lysosomal proteinases, have received more attention recently. Since elevated expressions of cathepsins and diminished levels of their inhibitors have been observed in several human cancers, including breast, gastric and prostate cancer, especially in aggressive cancer cells, cathepsins have been suggested to be biological markers of malignant tumors and have proved useful for prognosis of the disease. Furthermore, cathepsins have various roles in cancer progression. Cathepsin D has a mitogenic activity independent of its proteolytic activity and it attenuates the anti-tumor immune response of decaying chemokines to inhibit the function of dendritic cells. Cathepsins B and L have been shown to play an important role in matrix degradation and cell invasion. The administration of their inhibitors prevents the invasion and metastasis of cancer cells. These results indicate that cancer cells orchestrate various cathepsins to progress malignant diseases. Cathepsins may be a potential target for cancer therapy. J. Med. Invest. 52: 1-9, February, 2005