AbstractTo evaluate omega-3 fatty acid-rich enteral nutrient effects in patients with unresectable or recurrent biliary tract or pancreatic cancers during chemotherapy. Enteric nutritional supplements containing omega-3 fatty acids (Racol®) was administered to aforementioned patients with cancers during chemotherapy. The skeletal muscle mass and blood test data were obtained pre-administration and 28 and 56 days after. Patients with pancreatic cancer were administered the digestive enzyme supplement pancrelipase (LipaCreon®) 28 days after the start of Racol® administration. The number of chemotherapies skipped due to neutropenia was recorded for 2 months before and after enteral nutrient initiation. In all 39 patients, the skeletal muscle mass increased on day 56 versus baseline (median 17.3 kg vs. 14.8 kg, p < 0.01), number of chemotherapies skipped decreased (mean: 0.65 times/month vs. 1.3 times/month, p = 0.03), and retinol-binding protein (mean: 2.56 mg/dL vs. 2.42 mg/dL, p = 0.05) increased. Patients with pancreatic cancer showed increased blood eicosapentaenoic acid concentration on day 56 versus baseline (median: 48.1 μg/mL vs. 37.0 μg/mL, p = 0.04) and increased skeletal muscle mass (median 16.8 kg vs. 14.4 kg, p = 0.006). Baseline median neutrophil count increased significantly from 2200/μL at baseline to 2500/μL (p = 0.04). Patients with biliary tract cancer during chemotherapy also exhibited increased skeletal muscle mass following omega-3 supplementation (median 17.3 kg vs. 15.8 kg, p = 0.01). In patients undergoing chemotherapy for unresectable or post-recurrence pancreatic and biliary tract cancers, high-omega-3 fatty acid nutrition therapy use improved skeletal muscle maintenance and chemotherapy dosing intensity.