Oral mucositis (OM) is a complication of high-dose chemotherapy (HDC) which is frequently observed in hematopoietic SCT settings. Antioxidant agents have been proposed to prevent OM and therefore N-acetyl cysteine (NAC) could have an important role. In the present study, we conducted a double-blind, randomized, placebo-controlled study to evaluate the NAC effect on OM incidence and severity, and also glutathione peroxidase-1 activity. Leukemia patients undergoing allogeneic hematopoietic SCT preceded by HDC were recruited into the study and received either NAC (100 mg/kg/day) (n=38) or placebo (n=42) from the starting day of HDC until day +15 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 NAC group (23.7% vs 45.3%, P=0.04). Moreover, the mean duration of OM was significantly shorter in the intervention group (6.24(2.96) vs 8.12(3.97) days, P=0.02). The glutathione peroxidase-1 activity was also significantly higher in the NAC group seven days after transplantation (3.38(2.19) vs 2.41(1.70) ng/mL, P=0.003). It is concluded that parenteral NAC is effective in reducing the incidence of severe cases and the total duration of OM.