Oral mucositis (OM) is a complication of high-dose chemotherapy (HDC) followed by hematopoietic SCT (HSCT) with few effective treatments. Selenium has a cytoprotective role via the glutathione peroxidase (Glu.Px) enzyme and prevents chemotherapy-induced toxicities. We performed a double-blind, randomized, placebo-controlled study to evaluate the efficacy of selenium on the prevention of OM in 77 patients with leukemia, undergoing allogeneic HSCT. Thirty-seven patients received oral selenium tablets (200 mcg twice daily) from the starting day of HDC to 14 days after transplantation. OM was evaluated daily for 21 days after transplantation according to World Health Organization oral toxicity scale. The incidence of severe OM (grades 3-4) was significantly lower in the selenium group (10.8% vs 35.1%, P<0.05). We noted that the duration of objective OM (grades 2-4), excluding patient's self-declaration (grade 1), was significantly shorter in the selenium group (3.6±1.84 vs 5.3±2.2 days, P=0.014). Significant elevations in serum selenium level and plasma Glu.Px activity were observed 7 and 14 days after transplantation compared with baseline in the selenium group. We conclude that selenium can reduce the duration and severity of OM after HDC. Clinicaltrial.org ID: NCT01432873.