Killer cell immunoglobulin-like receptors (KIR) are highly polymorphic members of the immunoglobulin superfamily, which influence the response of natural killer cells and some T-lymphocyte subsets. Analysis of a cohort of previously human cytomegalovirus (HCMV)-negative patients, who developed primary HCMV infection following HCMV-positive renal transplant (n= 76), revealed an increase in the frequency of KIR genes located on the telomeric region of B haplotypes (Tel B). The presence of Tel B in combination with the KIR ligand HLA-C2 was significantly more frequent in this subgroup. These genetic factors were associated with resistance to HCMV infection in a second cohort (n= 65), where the Tel B genes KIR2DL5,-2DS1, 2DS5 and-3DS1 were all significantly associated with high viral loads. Furthermore, the KIR haplotype Tel A when in combination with the KIR ligand HLA-C1 was significantly protective against the development of severe infection. Our results suggest that KIR are a significant factor in the control of primary HCMV infection, and that determination of KIR gene repertoire may help in detection of renal transplant patients who were most at risk.