Cytomegalovirus (CMV) infection profoundly affects the T cell compartment and is associated with alterations in T cell aging parameters and generation of cytotoxic CD4þ CD28null T cells. Hence, the effect of a primary CMV infection post-kidney transplantation (KT) on the peripheral T cell compartment was examined. As aging parameters, we determined the T cell differentiation status, T cell receptor excision circle (TREC) content, CD31þ na€ ıve T cell numbers and relative telomere length (RTL) pre-KT and 12 months post-KT. CMV-seronegative KT recipients, receiving a kidney from a CMV-seropositive donor (Dþ/RÀ) were compared to Dþ/Rþ KT recipients. Eleven out of the 22 Dþ/RÀ KT recipients had CMV viremia post-KT. They developed CMV-specific CD4 þ and CD8 þ T cells and their T cell compartment shifted towards a more differentiated memory phenotype with expansion of CD4 þ CD28null and CD8 þ CD28null cells. One year post-KT, the CD8 þ T cell count was almost doubled compared to nonviremic Dþ/RÀ and Dþ/Rþ KT recipients. In addition, the RTL of the CD8 þ T cell was significantly lower and both the TREC content and CD31 þ na€ ıve T cell numbers significantly decreased. Moreover, primary CMV infection was associated with a negative impact on glomerular filtration rate. In conclusion, primary CMV infection has a substantial impact on the number and phenotype of peripheral T cells and may negatively affect renal allograft function.