Aim: To investigate whether there is a difference in selenium levels before and after radiotherapy (RT) and to study the effects of serum selenium levels on RT-related toxicity in patients undergoing RT for head and neck cancer. Patients and Methods: A population of 47 consecutive patients was enrolled in the study. RT was given by conventional fractionation. RT-related acute toxicity was evaluated once a week. Blood samples were obtained before and after RT to evaluate selenium levels. Results: There was no significant difference between the levels of selenium before and after RT (58.09 +/- 1.36 mu g/l and 56.34 +/- 1.11 mu g/l, p-value=0.747, respectively). Grade mucositis, dysphagia, radiodermatitis, and nausea were seen in 6 (12.7%), 32 (68.2%), 24 (51.1%), and 3 (6.4%) patients, respectively. It was found that there was no statistically significant difference in the levels of selenium before and after RT, and no observed diferrences in regard to RT-related toxicities. Conclusion: The serum selenium levels do not affect RT-related toxicities.