10024 Background: Children with cancer undergoing dose-intensive chemotherapy often require G-CSF support. Patients (pts) < 2 yrs have the highest risk of prolonged neutropenia due to imprecise dosing and immature drug clearance. Once-per-cycle pegfilgrastim may be beneficial due to neutrophil-mediated clearance that allows adequate serum levels throughout the neutropenic period. Young pts, who receive relatively more myeloablative chemotherapy, should have the highest pegfilgrastim exposure. Methods: Pts ≤21 yrs (age groups 0–5, 6–11, and ≥12 yrs) with sarcoma and scheduled to receive VAdriaC (cycles 1 & 3) and IE (cycles 2 & 4) were randomized 6:1 to receive pegfilgrastim 100 μg/kg or filgrastim (5 μg/kg/d until ANC ≥ 10×109/L) 24 hrs after chemotherapy administration. Post-nadir ANC recovery, incidence of ANC recovery by day 21 of cycles 1 & 3, pharmacokinetics (PK), and safety by age group were evaluated. Results: 43 pts (63% male) received pegfilgrastim (n=37) or filgrastim (n=6). Maximum pegfilgrastim concentration was achieved ∼24-hrs postdose and sustained until ANC nadir. Pegfilgrastim levels fell rapidly as ANC recovered. One child did not achieve ANC recovery by day 21 in cycle 1: an 8-mo-old 7.9-kg boy with non-metastatic undifferentiated sarcoma had 24 days of severe neutropenia in cycle 1. The youngest age group (including this child) had longer duration of severe neutropenia and, consequently, higher pegfilgrastim exposure ( Table ). Treatment-related adverse events were generally mild to moderate (pegfilgrastim 22%, filgrastim 33%); the only treatment-related event reported by >1 pt was bone pain (pegfilgrastim=11%, filgrastim=17%). Conclusions: ANC, PK, and safety profiles in pediatric patients were comparable with those in adults and support pegfilgrastim dosing across pediatric age groups. Consistent with neutrophil-mediated clearance, adequate exposure to pegfilgrastim was maintained in the youngest pts who received the most myelosuppressive chemotherapy. [Table: see text] [Table: see text]