PURPOSE: Recent studies have shown that the type 1/2 CD4+T cell (Th1/Th2) balance shifts toward Th2 dominance in cancer-bearing state or by surgical stress. Perioperative immunonutrition is reported to improve the outcome in patients with gastrointestinal cancer. This study was designed to investigate whether preoperative immunonutrition corrects the impaired Th1/Th2 balance in the perioperative period. METHODS: Thirty-six patients with colorectal cancer were prospectively divided into two groups as follows: preoperative oral intake supplementation with a formula enriched with arginine, ω-3 fatty acids and ribonucleic acid for five days (supplemented group; n = 19); and (control group;n = 17). Blood sampling was performed before supplementation, on the morning of surgery, and 3, 7, and 14 days postoperatively. The proportions of CD4+T cells producing intracellular cytokines (interferon-γand interleukin-4) were measured by flow cytometry. RESULTS: In the preoperative period, the proportions of CD4 +T cells producing interleukin-4 significantly decreased and Th1/Th2 ratio significantly increased on the morning of surgery compared with those before supplementation. In the postoperative period, the proportions of CD4 +T cells producing interferon-γin both groups maintained the preoperative level. The proportions of CD4+T cells producing interleukin-4 in the control group showed a gradual increase from the preoperative level, which implies Th2 dominant shift. In contrast, the supplemented group maintained the preoperative level of Th1/Th2 ratio. CONCLUSIONS: Preoperative immunonutrition corrects impaired Th1/Th2 balance in both cancer-bearing state and the postoperative period. This correction may be one of the important determinants of the clinical benefits of immunonutrition. PURPOSE: Recent studies have shown that the type 1/2 CD4 + T cell (Th1 / Th2) balance shifts toward Th2 dominance in cancer-bearing state or by surgical stress. Perioperative immunonutrition is reported to improve the outcome in patients with gastrointestinal cancer. This study was designed to investigate whether preoperative immunonutrition corrects the impaired Th1 / Th2 balance in the perioperative period. METHODS: Thirty-six patients with colorectal cancer were prospectively divided into two groups as follows: preoperative oral intake supplementation with a formula enriched with arginine, (control group; n = 17). Blood sampling was performed before supplementation, on the morning of surgery, and 3, 7, and 14 days postoperatively. The proportions of CD4 + T cells producing intracellular cytokines (interferon-γ and interleukin-4) were measured by flow cytometry. RESULTS: In the preoperative period, the propo rtions of CD4 + T cells producing interleukin-4 significantly decreased and Th1 / Th2 ratio significantly increased on the morning of surgery compared with those before supplementation. In the postoperative period, the proportions of CD4 + T cells producing interferon-γin both groups maintained the preoperative level. The proportions of CD4 + T cells producing interleukin-4 in the control group showed a gradual increase from the preoperative level, which implies Th2 dominant shift. In contrast, the supplemental group maintained the preoperative level of Th1 / Th2 ratio. CONCLUSIONS : Preoperative immunonutrition corrects impaired Th1 / Th2 balance in both cancer-bearing state and the postoperative period. This correction may be one of the important determinants of the clinical benefits of immunonutrition.