Administration of oral BU is a challenge in pediatric patients. The I.V preparation can improve compliance and eliminate variability in absorption. The metabolism and clearance of BU is age dependant. A prospective trial using a single daily dose of I.V. BU as part of a reduced Intensity Regimen was undertaken. Patients were enrolled after proper informed consent was signed... The regimen consisted of a test dose of BU (0.8mg/kg as a 2 hr infusion) on day -10, Fludarabine 25mg/m2/day from day-10 to -5, BU 3.2 mg/Kg/Day on days -5 ,-4.and rabbit ATG 2mg/kg daily on days -4 to-1. Cells were infused on day 0. The stem cell source were UCB (n=4), MUD (n=7) and MSD (n=1). PK samples (2, 4, 6,8,12 hrs) were obtained and submitted to the Seattle Cancer Care Alliance for determination of the Area Under the Curve (AUC) and clearance for BU. Twelve patients were enrolled (8F, 4M), median age 8 (0.5–16), median weight 22.6 kg (4.3–58.8) with the following diagnoses: Neuroblastoma (n=3), ALL (n=3), CML (n=2), Aplastic Anemia and X linked Lymphoprolipherative Disease, Ommen’s syndrome, Severe Combined Immune Deficiency. An AUC of 800–1200 and 3200–4800 uMol*min were targeted for the test dose and the single daily dose respectively. The median AUC for the test dose was 1003 uMol*min (804–1315), and the median clearance was 3.1 (ml/min)/Kg (2.4–7.3). The median AUC for the single daily dose was 3512 uMol*min (1511–4097) and the clearance 3.4 (ml/min)/kg (2.8–6.8). After the test dose 4 patients had their dose adjusted (2 patients lower and 2 patients higher). In the very young patients (< 1 year of age) the AUC of the single daily dose could not be predicted due to abnormal clearance. No instances of VOD or seizures were observed. Full donor chimerism was achieved in 8 patients in a median of 23 days (14–53), 2 patients developed graft failure, 1 patient was not evaluable for engraftment due to early TRM from pre-existing CMV pneumonia and 1 patient is too early. In conclusion, a single daily dose of I.V. BU is feasible, a test dose can be a predictor of the single daily dose AUC except in the very young, No regimen related toxicity was observed and engraftment was prompt and complete in 8 evaluable patients.