Background: Because all patients receiving bone marrow transplant (BMT) and peripheral blood progenitor cell transplant (PBPCT) experience gastrointestinal (GI) toxicity from the preparative regimen of chemotherapy, with or without radiation, oral glutamine was administered during the preparatory regimen and after transplant to maintain GI structure and function. Methods: To evaluate effects of oral glutamine on nutritional status and overall outcome, a prospective, randomized, double-blinded study was performed on 58 autologous and allogeneic BMT patients. Patients received 30 g of oral glutamine or placebo daily. Results: The trends of decreased median length of stay and the median number of days of total parenteral nutrition (TPN) were seen in the group supplemented with the >0.285-g/kg (the recommended dosage) dose of glutamine; however, there was no statistically significant difference in the nutritional status and overall patient outcome as assessed by days receiving TPN, number of days required until oral intake resumed, length of hospitalization, number of days and highest grade of mucositis, and quantity and number of days of diarrhea. Conclusions: This study does not support the hypothesis that oral glutamine may offer benefit. Further investigation is required regarding clinical tools for determining effectiveness, administration for tolerance and compliance, dosage, and potential of oral glutamine usage.