In the field of organ transplantation, overimmunosuppression is associated with severe side effects, such as infection, drug toxicity, and cancer, whereas underimmunosuppression is associated with acute rejection. Intracellular adenosine triphosphate (iATP) concentration following CD4 cell activation provides an assessment of cellular immune function to help monitor the immune status of immunosuppressed patients. This assay has shown to be the first post-transplant test related not only to the risk of acute rejection but also with the appearance of infection. The aim of our study was to compare the iATP concentrations of CD4 cells between healthy adults and kidney transplant recipients from a European population, analyzing the differences according to transplant clinical status. Samples from 81 kidney transplant patients who were admitted to our hospital over a nine-month period were drawn. T-cell activation was measured by determining the increase of iATP from CD4 cells. Results were compared with patient clinical status (rejection, infection, and stability). Three patients suffered an acute rejection episode and they were not included in the analysis (mean iATP concentration 247 +/- 87 ng/mL). iATP concentrations differed significantly between stable and infected patients (313 +/- 193 vs. 197 +/- 114 ng/mL; p = 0.008). iATP concentration values were not related to the length of admission, age, peak and current panel reactive antibodies, mismatches, leukocytes, weight, creatinine, days after transplantation and blood levels of cyclosporin, tacrolimus, and sirolimus. This assay measures global immune responses of CD4 T cells from a whole-blood sample, allowing for the assessment of the impact of immuno- suppressive drugs and of the patient's underlying clinical conditions. This assay identifies transplant patients at risk for infection or rejection, providing information which can guide immunosuppressive therapy.