Bone marrow transplantation (BMT) has become an important therapeutic modality in children with various immunologic, hematologic, and oncologic disorders. Multiple chemotherapeutic agents are used at high doses during the preconditioning period, and these agents often cause nausea and vomiting. Because of lack of data, this study was conducted to determine the efficacy and safety of ondansetron 0.15 mg/kg given intravenously every 4 to 6 hours in children undergoing BMT. A total of 105 treatment days were evaluated for 17 patients receiving ondansetron therapy. For all conditioning agents, complete response (no emesis) or major response (1 to 2 emetic episodes/d) was observed in 41% or 30% of treatment days, respectively; 18% had minor response (3 to 5 episodes/d), and 11% had failures (> 5 episodes/d). Ondansetron was well tolerated. Thus ondansetron 0.15 mg/kg every 4 to 6 hours appears to be effective and safe as an antiemetic drug in children during pretransplant conditioning with chemotherapeutic agents.