TS-1/CPT-11 treatment for the patients with advanced colorectal cancer is expected to be equal to FOLFOX and FOLFIRI regimen. The aim of this study was to investigate the relationship between host immunity and this regimen. Four patients were enrolled in this study. Host immunity was investigated before and after chemotherapy. PHA response, serum IAP level and the productions of any cytokines did not change significantly before and after the chemotherapy. Although these changes were not significant: both Th1/Th2 ratio and NK cells ratio were decreased, and the ratio of suppressor T cells was increased. These results suggest that TS-1/CPT-11 regimen may influence host immunity a little bit worse.