The effects of n-6 and n-3 polyunsaturated fatty acids (PUFA) on protein metabolism, cell-mediated immunity, and production of cytokines and prostanoids were studied in experimental animals and patients with esophageal cancer. In the experimental study using a rat burn model, n-6 PUFA increased serum interleukin-6 (IL-6) and tumor necrosis factor (TNF), alpha (P < 0.05), and decreased nitrogen balance (NB) (P < 0.05), when compared with a fat-free control. But addition of n-3 PUFA reduced TNF-alpha and IL-10 (P < 0.05) and improved NB (P < 0.05). Suppressed delayed type hypersensitivity (DTH) induced by burn injury, which was not influenced by n-6 PUFA, was significantly improved by the administration of n-3 PUFA. n-6 PUFA tended to increase, and n-3 PUFA significantly decreased the endotoxin translocation. DTH, granulocyte-macrophage colony-stimulating factor, and eicosapentaenoic acid (EPA) content increased proportionately with the intravenous dose of fish oil emulsion. The effects of n-6 and n-3 PUFA were studied in the patients who underwent surgery for esophageal cancer. In the group of patients fed by total parenteral nutrition with soybean oil emulsion, the serum IL-6 significantly increased at 2 and 6 h after operation (P < 0.05). Oral/enteral supplementation of EPA ethyl ester (1.8 g/d) significantly reduced the postoperative IL-6 production (P < 0.05 at 1, 2, and 6 h after operation), and improved cell-mediated immune function 3 wk after operation (P = 0.05). During the chemoradiation therapy, cell-mediated immune function was improved significantly in the patients fed enterally with EPA ethyl ester (n = 5), when compared with the patients without EPA (n = 14).