Hemorrhagic cystitis is a potentially serious complication of high-dose cyclophosphamide therapy administered before bone marrow transplantation. As standard practice at our institution, patients who are scheduled to receive a bone marrow transplant are treated prophylactically with forced hydration and bladder irrigation. In an attempt to obviate the inconvenience of bladder irrigation, we conducted a feasibility trial of uroprophylaxis with mesna, which neutralizes the hepatic metabolite of cyclophosphamide that causes hemorrhagic cystitis. Of 97 patients who received standard prophylaxis, 4 had symptomatic hemorrhagic cystitis. In contrast, two of four consecutive patients who received mesna uroprophylaxis before allogeneic bone marrow transplantation had severe hemorrhagic cystitis for at least 2 weeks. Because of this suboptimal result, we resumed the use of bladder irrigation and forced hydration to minimize the risk of hemorrhagic cystitis.