The proliferation potentials and the level of apoptosis were compared in paired primary colorectal adenocarcinomas and their liver metastases within each individual. From a total of 22 patients 44 specimens of paired primary and metastatic tumors were obtained for analysis. The levels of spontaneous apoptosis (a spontaneous apoptosis index, SAI: % apoptotic nuclei among a total of 1000 nuclei) and of proliferation (KI-67 index: % positively stained cells for KI-67 among a total of 1000 cells) were analyzed between primary and metastatic tumors. Survival rates and its relationship with the clinical parameters were also analyzed. The overall survival rate at 5 years was 16.9% with the median survival time of 45 months. T-stage (p=0.005) and time to liver metastasis (synchronous versus metachronous, p=0.03) showed statistical significance in relation to survival. The mean SAI of primary tumors was 1.35 ± 0.25, which was not statistically different from the 1.58 ± 0.18 of metastatic tumors (p=0.33). The mean KI-67 indices in primary and metastatic tumors were 23.9 ± 3.4 and 16.4 ± 2.5, respectively, and this difference was statistically significant (p=0.016). Subset analysis showed significant difference in the KI-67 index in the synchronous group but not in the metachronous group. No significant difference was shown in the relative ratios of apoptosis to proliferation between the primary tumor and the metastasis within each individual. The results in this study may partly explain the indolent behavior of liver metastasis from colorectal cancer and provides a rationale for the active treatment of metastatic tumors as well as of primary disease.