Specific NK cell killer inhibitory receptor (KIR):HLA haplotype combinations have been associated with successful clearance of acute and chronic HCV infection. Whether an imbalance of activating NK cell receptors also contributes to the outcome of treatment of chronic HCV infection, however, is not known. We studied peripheral NK cell phenotype and function in 28 chronically viraemic HCV genotype I treatment-naïve patients who underwent treatment with pegylated IFN-α and ribavirin. At baseline, chronically infected patients with sustained virological response (SVR) had reduced CD56brightCD16+/− cell populations, increased CD56dullCD16+ NK cell proportions, and lower expression of NKp30, DNAM-1, and CD85j. Similarly, reduced NK cell IFN-γ production but increased degranulation was observed among nonresponding (NR) patients. After treatment, CD56brightCD16+/− NK cell numbers increased in both SVR and NR patients, with a parallel significant increase in activating NKp30 molecule densities in SVR patients only. In vitro experiments using purified NK cells in the presence of rIL-2 and IFN-α confirmed upregulation of NKp30 and also of NKp46 and DNAM-1 in patients with subsequent SVR. Thus, differences in patient NK cell receptor expression and modulation during chronic HCV-1 infection are associated with subsequent outcome of standard treatment. Individual activating receptor expression/function integrates with KIR:HLA genotype carriage to determine the clearance of HCV infection upon treatment.