Chemo (chemoradio) therapy can induce oral mucositis and change body composition in patients with esophageal cancer. The impact of the amino acid-rich elemental diet Elental® on oral mucositis and changes in body composition during chemo (chemoradio) therapy is unclear. Thus, the purpose of the present study was to examine the preventive effects of Elental on oral mucositis and sarcopenia progression during chemo (chemoradio) therapy for esophageal cancer. Patients were randomized to receive either azulene oral rinse (Arm 1) or Elental (Arm 2) during the treatment cycle (4 weeks). The incidence of oral mucositis and other adverse events was evaluated weekly. Body composition pre- and post-treatment cycle was measured by bioelectric impedance analysis. Thirty‑three patients (17 azulene and 16 Elental) completed the study, and the groups were well matched. Elental tended to reduce the incidence of oral mucositis (Arm 1, 23.5% and Arm 2, 12.5%), but there was no statistically significant difference between the groups. The average body mass index (BMI) and body fat mass decreased significantly in both groups after the treatments. Lean body mass (LBM) was reduced in Arm 1, but was increased in Arm 2 after the treatment; the relative change of LBM after the treatment was significant between Arm 1 and Arm 2 (P=0.007). This study revealed that Elental nutrition could counteract sarcopenia development during chemoradiotherapy for esophageal cancer. These properties may lead to improvement of the quality of life and clinical outcome of esophageal cancer patients treated with chemo (chemoradio) therapy (Clinical Trial Registry ID: UMIN 000007960).