SUMMARY In the present study, we evaluated the NK cell cytotoxic activity in a group of HCV-infected individuals. Although the number of NK cells present in the peripheral blood of the HCV-infected patients was comparable to non-infected individuals, spontaneous NK cytotoxicity was four-fold lower (P < 0.001) than in normal donors. This functional impairment was not overcome by depletion of adherent or B cells, and it was partially restored by short-term (18 h) stimulation with IL-2. However, long-term stimulation (72 h) with this lymphokine induced activated killer cell (LAK) activity comparable to normal controls. The reduction in NK cytotoxic response does not seem to be due to soluble suppressive factors, since incubation of normal peripheral blood mononuclear cells (PBMC) with infectious HCV serums for a 4-h period does not affect NK spontaneous cytotoxic activity. Successful in vitro infection of PBMC with HCV infectious serum also resulted in an impairment of NK cytotoxicity, suggesting that altered NK function is associated with HCV infection and may be responsible, at least in part, for the chronicity of the infection.