Concurrent Chemoradiotherapy (CCRT) induces toxicities from inflammation and immunological suppression. Omega-3 fatty acids, glutamine, and arginine are therapeutic factors that can attenuate such inflammation and promote cellular immunity. The question is whether immunonutrition (IN) during CCRT reduces inflammation and improves the immune function in patients with esophageal squamous cell carcinoma (ESCC). Seventy-one locally advanced ESCC patients being treated with CCRT (5-FU and cisplatin) were randomized into 2 groups. The IN group received a combination of omega-3 fatty acids, glutamine, and arginine , whereas the control group received standard formula. The levels of C-reactive protein (CRP), tumor necrosis factor (TNF), interferon-gamma (IFN), interleukin (IL-6, IL-10), CD3, CD4, CD8, white blood cells, neutrophils, and total lymphocytes were measured before and during treatment. The levels of CRP (p = 0.001) and TNF (p = 0.014) increased more during treatment in the control group than the treatment group, whereas IFN, IL-6, and IL-10 were similar but not significantly. CD3, CD4, CD8, white blood cells, neutrophils, and total lymphocytes decreased more in the control group than in the treatment group, but not significantly. Enteral IN during CCRT reduced the increase of inflammatory cytokine levels.