Background: The concept of immune cell exhaustion in the context of metastatic melanoma has been reinforced by the success of immunotherapies targeting the exhaustion markers CTLA-4 and PD-1. Natural Killer (NK) cell exhaustion, characterized by an up-regulation of inhibitory receptors and loss of function, was described in the context of melanoma. Ipilimumab (IPI - anti-CTLA-4) improves the anti-tumor T cell activity and achieves response rates of 15-20%, however the effect of IPI on NK cells is unknown. In this project, we studied the effect of IPI on the phenotype of NK cells from melanoma patients and how it relates with clinical response. Methods: NK cells were purified from the peripheral blood of 10 advanced melanoma patients treated with IPI. Blood samples were collected at baseline, after the 2^nd and 4^th cycles of IPI. NK cells were characterized acccording to the expression of activating (NKG2D) and inhibitory (KIRB1) receptors, function (cytotoxicity, IFN-γ production), levels of the IL-2R α chain and response to IL-2 stimulation. We analyzed the effect of IPI on NK cells as it relates to clinical response. Results: IPI induces an upregulation of 50% in the IL-2R α chain levels on NK cells (p = 0.03). There was no significant difference in other receptors or function (p > 0.05). We then checked the phenotype and function of these NK cells after 48 hours of IL-2 stimulation and in 4 patients out of 10 there was an improvement of cytotoxicity and higher levels of IL-2R α chain. Remarkably, this better response to IL-2 was observed in patients with clinical response to IPI (partial/complete response) compared with non-responders, with higher cytotoxicity (p = 0.05) and levels of IL-2R α chain (p = 0.02). Conclusions: CTLA-4 is expressed mainly on T cells, including regulatory T cells, with no expression on NK cells. Nevertheless, we have shown an effect of IPI on the phenotype of NK cells, with an increase of IL-2R expression. More importantly, the effect of IPI on NK cells (better response to IL-2 stimulation and cytotoxicity) is associated with a good clinical response. The mechanism behind this effect is not clear yet, however this may be indirect through the action of IPI on other immune cells.