Lifelong immunosuppression is mandatory for optimal graft and patient survival following liver transplantation. Nevertheless, graft rejection or numerous adverse events associated with overimmunosuppression or underimmunosuppression cannot be completely avoided. The ImmuKnow assay measures cell-mediated immunity and is able to discern between conditions of overimmunosuppression and underimmunosuppression. The aim of this study was to evaluate the ImmuKnow assay in the evaluation of the immune function in pediatric liver transplant recipients and to assess its correlation with the patients' clinical and biochemical status. Eighty-nine whole blood samples were collected from 23 liver transplant recipients that were 1 to 18 years old. The net state of immune function was determined by the quantitative measurement of the intracellular adenosine 5-triphosphate level in CD4+ lymphocytes after phytohemagglutinin stimulation. Comprehensive clinical data were correlated with the ImmuKnow assay results. In 23 of the 28 samples collected during clinical quiescence, ImmuKnow results were correlated with the clinical status, expressing the patient's moderate immune function. However, a correlation between measured therapeutic drug levels and clinical quiescence was found in only 18 of the 28 samples. In 6 patients who suffered from clinical complications, ImmuKnow measurements showed a wide range of deviations, expressing the unstable immunological status of these patients. In conclusion, the ImmuKnow assay correlates with the clinical status of liver-transplanted children. It serves as a reliable and unique parameter of the cellular immune function. We conclude that the ImmuKnow assay, together with existing clinical tools, may allow for the immune monitoring of pediatric liver recipients. Liver Transpl 14:893–898, 2008. © 2008 AASLD.