A 16-year-old boy experienced a severe macroscopic hemorrhagic cystitis 20 days after a partially mismatched allogeneic marrow transplantation for an acute lymphoblastic leukemia. The conditioning included cyclophosphamide and total body irradiation. After a conservative management, the hematuria spontaneously resolved after 3 months of gross hematuria. The authors review the literature, and stress the predisposing factors and clinical outcome of such complication. They advocate a conservative "watch-and-wait" approach including intensive bladder irrigation.