We studied 30 pediatric patients with malignant (n = 16) or nonmalignant (n = 14) conditions. The preparative regimen consisted of fludarabine, intravenous (IV) busulfan (Bu) for 2 daily doses, and antithymocyte globulin before stem cell transplantation. A test dose of IV Bu (0.8 mg/kg), anticipated to target an area under the concentration-time curve (AUC) of 800 to 1200 micromol.min, was followed later by 2 daily doses adjusted according to the pharmacokinetics (PK) to target an AUC of 3200 to 4800 micromol.min. The median test dose AUC was 953 micromol.min (range, 439-1315 micromol.min). The median AUC of single daily doses was 3798 micromol.min (range, 1511-7254 micromol.min). PK-based dose modification was required in 20 patients: 12 were adjusted to a higher dose, and in 8 the dose was decreased. Nausea and vomiting were noted in 15 patients. No patient developed hepatic veno-occlusive disease or seizures. Full donor chimerism was attained in 20 patients (mean of 24.5 days), 3 achieved partial chimerism, 5 did not engraft, and in 2 it is too early to assess chimerism. Acute graft-versus-host disease developed in 11 patients, grades I to II developed in 10 patients, and grade III developed in 1. Four patients died of infection and 5 of progressive disease. Thus, PK of a test dose of IV Bu provided information to adjust subsequent daily doses of IV Bu: this resulted in a regimen that was feasible, safe, and convenient for administration to children.