Monitoring CSA levels at two h after dosing (C2) has been shown effective in providing adequate CSA-based immunosuppression in clinical trials in adult transplant recipients, but there is limited data regarding C2 monitoring in pediatric transplant recipients. Given the differences in CSA pharmacokinetics between children and adults, a cohort of stable pediatric transplant recipients was converted from monitoring CSA trough (C0) to C2 levels, to establish the clinical utility and safety of C2 monitoring. After an abbreviated AUC 0-5 to establish baseline exposure, subsequent CSA dosing was adjusted based on C2 levels. Additional evaluation included monitoring for rejection, changes in CSA dose, toxicity, serum chemistries, and infection. Twelve heart transplant recipients were enrolled, with mean age 4.8 yr (range: 0.6-14.0). All patients received microemulsified CSA (Neoral®; Novartis Pharmaceuticals, East Hanover, NJ, USA) twice daily. Baseline CSA dose was 5.39 ± 2.05 mg/kg/day (mean ± s.d.), with mean C0 = 267 ± 112, C2 = 1065 ± 565, and AUC0-5 = 3817 ± 1435. Only seven participants showed clear CSA peak levels at two h, with five exhibiting delayed peaks at three to five h post-dose. These seven participants completed 48 wk of study, with mean CSA dose decreasing to 4.55 ± 3.61 mg/kg/day, maintaining mean C2 599 ± 211 (vs. target C2 = 800). No significant change in serum creatinine was observed, although GFR increased from 76.9 to 107.6 mL/min/1.73 m2 (p = 0.11). Five patients failed to achieve target C2 levels (>800) during the first four wk, despite comparable AUC values, and were maintained on trough monitoring (C0). Mean systolic and diastolic blood pressures fell slightly, three minor infections were noted during the study period, and one episode of acute rejection occurred, despite stable CSA dosing. Nearly 50% of stable pediatric transplant recipients failed to achieve adequate peak C2 CSA levels during conversion from C0 to C2 monitoring. Age-dependent differences in CSA absorption and/or clearance pharmacokinetics may explain these findings.