Published data suggest that the average concentration of busulfan at steady state (Bu Css) is critical for successful engraftment in children receiving busulfan as a conditioning agent for bone marrow transplantation (BMT). We previously found in children that a Bu Css <600 ng/ml correlated with autologous recovery/mixed chimerism; there was no correlation between Bu Css and regimen-related toxicity (RRT). In a cohort continuous with the previous trial, we prospectively evaluated targeted busulfan concentrations in 32 pediatric patients (age 0.6-18.5 years) with AML (n = 6), CML (n = 6) and non-malignant disorders (n = 20) receiving HLA-closely matched donor grafts. In this trial, individual busulfan pharmacokinetics were performed prior to admission. Busulfan doses were then adjusted to achieve a Bu Css target range of 600-900 ng/ml +/- 10% depending on donor source and disease. A repeat study was done following dose 1 of the conditioning regimen. Thirty of thirty-two (94%) patients achieved target concentrations. Total busulfan doses ranged from 10.9 to 29 mg/kg. Thirty of thirty-two patients (94%) have durably engrafted. Grade 3/4 RRT occurred in seven patients (21%). Targeting Bu Css ranges of 600-900 ng/ml significantly improved our rate of successful engraftment from 74% to 94% (P = 0.043). These results indicate that targeted busulfan dosing optimizes allogeneic engraftment in children.