A pathogenetic role of natural killer (NK) cells has been hypothesized in spondyloarthritides (SpA), but still conflicting data exist. Aim of this study was to investigate, in a cohort of SpA patients, the peripheral blood NK cell number, the cytokine production (IFNγ and TNFα), and the cytotoxic activity on target cell stimulation. Moreover, we aimed to evaluate the expression of KIR3DL1, an inhibitory receptor binding HLA class I alleles, including HLA-B27 strongly associated with SpA, between different NK cell subsets.