Quantification of nuclear DNA content in cancer is considered a prognostic factor, and several investigators have demonstrated that aneuploid tumors are more aggressive than diploid tumors. In this report, we investigated the relationship between metastatic capacity and nuclear DNA content using gastric and colon cancer cell lines. We established a highly peritoneal metastatic cell line (2M-D3) from a gastric cancer cell line (2M) and a highly liver metastatic cell line (LM-H3) from colon cancer cell line (LM) in our laboratory. Both the highly metastatic cell lines have more capacity for growth and metastasis, but lower nuclear DNA content and chromosome numbers than these parental cell lines. The results indicated that metastatic capacity was inversely proportional to nuclear DNA content. We cannot provide a satisfactory explanation for this inverse correlation between metastatic capacity and nuclear DNA content, but it is supposed that each cancer cell may have suitable and stable nuclear DNA content in metastasis.