AIM OF THE STUDY This study explores the efficacy of oral glutamine in the prevention of acute radiotherapy-induced esophagitis in patients with lung cancer who are treated with thoracic radiotherapy. MATERIAL AND METHODS This study was planned as a retrospective randomized experimental study. Forty-six patients with lung cancer, who were treated and kept under control between January 2008 and January 2010, were included in the study by the Department of Radiation Oncology, Faculty of Medicine, Dicle University. The patients were divided into two groups. The first group (n = 21) was given prophylactic oral powder glutamine (daily 30 g), while the second group (n = 25) was not given oral glutamine. RESULTS There were 21 patients in Group 1 (45.7%) and 25 patients in Group 2 (54.3%). No significant statistical difference was observed between the two groups in terms of age, gender, stage, histopathological type, treatment choice, received radiation doses, esophagus length in RT field, or location of the tumor (p > 0.05). A significant statistical difference was observed between the glutamine-supplemented group (first group) and the glutamine-free group (second group) according to the grade of esophagitis (p < 0.0001). CONCLUSIONS In our retrospective randomized experimental study, we determined that the severity of acute radiotherapy-induced esophagitis might be decreased with oral glutamine in patients with lung cancer who were treated with thoracic radiotherapy.