CMV infection and disease remain a significant clinical challenge for pediatric solid organ transplant (SOT) recipients. Current prevention strategies are limited to prophylaxis in which antiviral medication is administered for a period of several months or preemption in which close monitoring of CMV viral load from the peripheral blood is performed and treatment is initiated when CMV is detected. Each of these strategies has risks, costs, and limitations associated with it. Recently, assays for measurement of an individual patient's CMV immunity have been developed and are clinically available. One of these is the Viracor CMV T cell Immunity Panel. This flow cytometry based assay is performed on peripheral blood and measures cytokine release in response to CMV antigen stimulation by flow cytometry. The thresholds for this assay that confer protection against CMV infection in pediatric SOT recipients are not known. Defining CMV-specific cell mediated immune response thresholds that confer protection against CMV reactivation could inform patient specific durations of antiviral prophylaxis or pre-emptive surveillance testing. Therefore, the objective of this study is to quantify CMVresponsive T lymphocyte populations by flow cytometry (Viracor CMV T cell Immunity Panel) in pediatric heart, kidney, and liver transplant recipients within the first year of transplantation and to investigate potential threshold values that correlate with protection against CMV infection (DNAemia).