Childhood obesity has more than tripled in the past 30 years. The prevalence of overweight and obese children has also increased in the pediatric cancer setting, causing substantial concern over proper chemotherapeutic dosing in this population. The purpose of this study was to determine if children with an increased body mass index (BMI) have an alteration in busulfan pharmacokinetics during hematopoietic stem cell transplant (HSCT) conditioning. We retrospectively reviewed data on busulfan pharmacokinetics (PK) on HSCT subjects (subjects were part of a prospective study previously reported by our group at Children's Memorial Hospital) to determine appropriateness of dosing. Subjects were divided into appropriate BMI categories (<25th percentile, 25th-85th percentile, >/= 85th percentile) and busulfan PK dosing was analyzed (test dose, regimen dose, area under the curve [AUC], and clearance). The dosing based on PK test dose data of children with BMI >/= 85% was compared against the package insert dosing recommendations of using adjusted ideal body weight (AIBW) in obese patients to determine which dosing schema was most accurate. Children with high BMIs had higher AUCs when dosing on actual weight then their normal or low BMI counterparts. This indicates that children with a high BMI require less drug (2.9 mg/kg using actual body weight) to achieve the same AUC as children with normal BMI (4.0 mg/kg) or low BMI (3.6 mg/kg). Using the recommended AIBW dosing schema, 53% of the patients with high BMIs would have had regimen dose AUCs >/= 20% over/under the target; whereas with the PK test dose method, only 16% of the patients with high BMIs had regimen dose AUCs >/= 20% over/under the target. PK testing continues to be the gold standard for busulfan dosing in children. Particular vigilance should be paid to PK monitoring in high BMI categories because of the potential risk of imprecise dosing when using the AIBW schema.