Ornithine decarboxylase (ODC), a critical regulatory enzyme for polyamine biosynthesis, is strictly regulated in human cells. Several studies suggested the importance of elevated enzymatic activity and altered biochemical characteristics of ODC in malignant cells. Because mutation of ODC in primary human hepatocellular carcinoma has been reported, we examined whether the genetic alterations, such as mutations or structural alterations of the gene, also account for the alteration of ODC activity in human colorectal cancer. No mutation or structural alteration in the ODC was detected in any of the colorectal tumors and normal tissues examined. These results suggest that a mutation or structural alteration of the ODC may not be involved in human colorectal carcinogenesis.