INTRODUCTION:Radiation-induced oral mucositis (RIOM) is one of the most common complications in head and neck cancer (HNC) patients and is thought to occur through oxidative stress. Selenium(Se) has attracted tremendous interest because of its importance as an antioxidant and antiinflammatory agent. OBJECTIVES: Evaluating the effectiveness of Se in the prevention of radiation-induced oral mucositis(OM). PATIENTS AND METHODS:A randomized controlled clinical study was conducted on forty head and neck cancer patients undergoing radiotherapy at the Department of Clinical Oncology, Faculty of medicine, Alexandria University, Egypt. Patients were assigned , equally, to either control group who received conventional preventive medications or test group who received 200 mg of selenium twice daily along with the conventional preventive medications. All patients were clinically evaluated for OM severity (objective assessment) and pain (subjective assessment) at day 1 of radiotherapy, day 14, day21 and day 42. RESULTS: There was no significant difference comparing control and test groups regarding severity of OM and mean pain score during the four times of assessment. CONCLUSION: The administration of Se supplementation during radiotherapy has no effect on OM severity and its related pain when assessed by World Health Organization (WHO) scale and visual analogue scale(VAS) ,respectively.