Liver and lung metastases adversely affect prognosis after surgery for colorectal cancer. However, few studies have compared the biological aggressiveness of primary tumors and metastases. The purpose of this study was to investigate the biological aggressiveness of colorectal liver and lung metastases. DNA ploidy and S-phase fraction (SPF) of liver and lung metastases were examined by flow cytometry and were correlated with the disease-free interval (DFI) after primary surgery and survival after metastasectomy in 77 patients with liver metastases and 34 patients with lung metastases from colorectal cancer. There were no significant differences in the rates of DNA aneuploidy between primary tumors and liver metastases (74.6% vs. 75.5%) or between primary tumors and lung metastases (62.l% vs. 73.9%). Similarly, there were no significant differences in the mean SPF of primary and metastatic tumors. The SPF of DNA aneuploid tumors correlated inversely with the DFI for both liver and lung metastases, however, the SPF of DNA diploid tumors correlated inversely with the DFI only in patients with liver metastases. Five-year survival rates after metastasectomy were 42.7% for patients with liver metastases and 47.8% for patients with lung metastases. The biological aggressiveness of colorectal cancer does not change after metastasizing to the liver or the lungs. Although DNA ploidy and SPF do not correlate with prognosis after the resection of liver or lung metastases, the SPF does predict the time to recurrence in patients with DNA aneuploid colorectal carcinomas.