Concurrent chemoradiotherapy, a standard therapy for locally advanced head and neck cancer, induces severe oral and pharyngeal mucositis which occasionally impede treatment schedules. To date, however, no drug has been approved or proven effective in the prevention and care of mucositis. In contrast, prophylactic percutaneous endoscopic gastrostomy (PEG) is well known for its ability to maintain the nutritional condition of patients and improve treatment compliance. Here, we retrospectively investigated whether two different nutritional diets would lead to different treatment outcomes. The patients had locally advanced head and neck cancer and had started definitive [check] chemoradiotherapy between 1 July and 31 December 2011, with PEG in advance. After exclusion of 6 patients who did not require nutritional support, 34 patients were included in the analysis. They were divided into two groups, one given the low-residue diet (LRD; n = 20) and the second the enteral diet (ED; n = 14), and compared for the rate of infection and mucositis, body weight loss, and treatment compliance. The infection rate was 30% in the LRD group and 7.1% in the ED group, while that of mucositis was 25% and 50%, respectively. Average glutamine intake per day was 4.8 g in the ED group, because ED includes this amino acid. Glutamine prevents infection in the fields of bone marrow transplantation and intensive care, and might also do so in the present patients. A better understanding of the role of glutamine and ED in nutritional support and the prevention and treatment of mucositis and infection awaits prospective investigation.