Objectives: This study analyzed the relationship of variability in routine trough cyclosporine (CSA) levels to morbidity after pediatric cardiac transplantation. Background: Due to high interindividual variation between dosage and blood concentrations, trough surveillance CSA levels are routinely performed after cardiac transplantation to adjust dosages. In addition, trough CSA levels have been used as a measure of patient compliance in transplant recipients. Recent investigations have demonstrated a relationship between late rejection and mistimed CSA dosing intervals, which could also lead to CSA levels that are incorrectly presumed to be trough levels. Methods: Trough surveillance whole-blood CSA levels were retrospectively reviewed in 49 pediatric heart transplant recipients who had a median follow-up of 42 months (range 6 to 138 months). All patients received the same immunosuppression regimen (CSA, azathioprine, and steroids), the same CSA-level surveillance protocol, and the same stabilization of CSA dose and level in the therapeutic range (150 to 300 ng/ml) prior to hospital discharge. CSA levels drawn because of coexisting phenomena (drug interaction, gastroenteritis) that could cause CSA-level fluctuation were excluded from analysis. Cyclosporine variability was measured as the percentage of CSA levels that were considered sub-therapeutic (≤ 100 ng/ml), toxic (≥ 450 ng/ml), or both. Cyclosporine-level variability was then analyzed in respect to demographic and outcome variables. Results: For the group, the median percentage of sub-therapeutic levels was 3% (range, 0% to 16%); the median percentage of toxic levels was 5% (range, 0% to 36%); and the median of the combination of sub-therapeutic and toxic levels was 10% (0% to 38%). Eight of the 49 patients (16%) had a high (> 20%) percentage of sub-therapeutic + toxic levels or high CSA variability. High CSA variability was significantly associated with recipients > 12 months of age (p = 0.05), black recipients (p = 0.009), recipients from single-parent households (p = 0.028), and recipients with a history of non-compliance (p < 0.001). Patients with high CSA variability had a significantly higher median number of hospitalized days per year of follow-up (p = 0.036), higher rate of recurrent rejection (≥ 2 episodes; p = 0.0003), and higher death rate more than 6 months after transplant (p = 0.01). Conclusion: Although this study could not determine cause, high variability in trough CSA levels was a marker for pediatric heart transplant recipients at greater risk for recurrent rejection and hospitalization after transplantation. Copyright (C) 2000 International Society for Heart and Lung Transplantation.