We have investigated the expression of matrix metalloproteinase 9 (MMP-9) and tissue inhibitor of metalloproteinases 1 (TIMP-1) in colorectal cancer by the immunostaining (avidin-biotin peroxidase complex method) and in situ hybridization (ISH). Both MMP-9 enzyme and messenger RNA (mRNA) for MMP-9 were located in tumor cells, neutrophils, monocyte-macrophages and fibroblasts in colorectal cancer tissue. The location of TIMP-1 mRNA was similar to that of MMP-9 mRNA in colorectal cancer tissue. There was a strong correlation between the expression of MMP-9 in tumor cells and liver metastasis. The expression of mRNA for TIMP-1 in stromal cells in cases associated with liver metastasis was significantly higher than that in cases without liver metastasis. However, in tumor cells, predominant expression of MMP-9 mRNA was observed in all cases associated with liver metastasis. These results suggest that MMP-9 might play an important role in hematogenous metastasis in colorectal cancer and that the balance between the production of MMP-9 and TIMP-1, in particular in tumor cells, is important as one of the pathogenesis of tumor metastasis.