Preventive effects of hemorrhagic cystitis by the use of sodium 2-mercaptoethane sulfonate (MESNA) was evaluated in 40 children undergoing peripheral blood stem cell autografts (PBSCT) after marrow-ablative chemotherapy which included high-dose cyclophosphamide (CY, 50mg/kg x 2). Fifteen patients received MESNA (group A) and 25 did not (group B), and all received concomitant hyperhydration (3,000ml/m2/day). No renal dysfunction or toxicity attributed to the use of MESNA was observed in either group of patients. Transient hemorrhagic cystitis developed in one of the 15 group A (6.7%) and 3 of 25 (12.0%) group B patients but there was no statistical significance. Although the results may suggest that MESNA is a controversial agent in preventing hemorrhagic cystitis caused by CY when hyperhydration protocol is used, further observation with a larger number of patients is required to establish a firm conclusion.