Natural killer (NK) cells are lymphocytes of the innate immune system. In humans, NK cell activities are partly controlled by the diverse killer immunoglobulin-like receptor (KIR) gene family. The importance of NK cells in both immunity to infection and reproduction makes KIR strong candidates for genes undergoing dynamic evolution in the human genome. Using high-resolution allelic typing, we investigated the potential role of natural selection in the diversification of KIR in the Irish population. Higher diversity than expected is observed at several loci, consistent with a history of balancing selection acting to maintain several allelic variants at high frequency in the population. KIR diversity is enhanced further at the haplotype level with functional polymorphisms at KIR2DL4, KIR3DL1 and KIR2DS4 defining nine 'core' haplotypes. Analysis of these core haplotypes in combination with human leukocyte antigen (HLA) class I ligands revealed several nonrandom associations. In particular, the KIR:HLA association for the core haplotype defined by KIR3DL1(*)01502 was female specific and a likely consequence of negative selection acting against KIR3DL1(*)01502 on an HLA-C1/C1 background. Many of the associations between KIR and HLA in the Irish differ from those previously reported, which argues against universal selective pressures for specific KIR:HLA combinations in diverse human populations.