Busulfan (BU) is a widely used myeloablative and antineoplastic agent in clinical bone marrow transplantation (BMT). The lower incidence of BU-associated toxicities and lower therapeutic effectiveness in young children given BU doses based on body weight (ie, 16 mg/kg) is associated with altered pharmacokinetics of BU; the area under the curve (AUC) of BU concentration versus time is significantly less in these patients than those observed in older children and adults. To optimize BU dosage in young BMT recipients, we developed a dosage regimen based on body surface area (BSA) and determined BU pharmacokinetics and BU-associated toxicities. Seven children (median age, 3.9 years, range, 1.1 to 5.7) undergoing allogeneic or autologous BMT for leukemia received 40 mg/m2/dose BU every 6 hours for 16 doses; BU concentrations were measured in the plasma, and AUCs were determined for each patient after the first and 13th doses. Expressed as a function of body weight, the median BU dosage was 26.4 mg/kg (range, 24.3 to 28.2), a 60% increase over the BU dosage based on body weight. Four patients developed mucositis, and one of them also developed nonfatal hepatic veno-occlusive disease (VOD). No patients receiving 40 mg/m2 BU developed neurotoxicity (eg, seizures) or interstitial pneumonitis. Prompt and sustained engraftment was observed in the allogeneic BMT recipients, and late graft failure was not seen. The mean BU AUCs were 1,105 mumol/L.min (range, 790 to 2,080) after the first dose and 1,022 mumol/L.min (range, 632 to 1,860) after the 13th dose of BU, comparable to the AUCs in adults given 16 mg/kg of BU. These studies suggest that, in young children, BSA-based dosing of BU (40 mg/m2) provides drug exposures (AUCs) closer to adult values with acceptable toxicities and may improve therapeutic effects.