In colorectal cancer, low-dose Leucovorin plus 5-FU therapy does not induce any severe adverse effects, and patients could receive this therapy for many cycles. The aim of this study was to investigate the relationship between host immunity and this regimen. Seven patients were enrolled in this study. Host immunity was investigated before and after chemotherapy. The ratio of Th1/Th2, PHA response, serum IAP level and the productions of IL-6, IL-10 and soluble IL-2 receptor did not change significantly before and after chemotherapy. Since low-dose Leucovorin plus 5-FU therapy might not influence host immunity, patients could receive the therapy for a long duration without toxicity.