NK cells are multicompetent lymphocytes of the innate immune system with a central role in host defense and immune regulation. Studies in experimental animal models of multiple sclerosis (MS) provided evidence for both pathologic and protective effects of NK cells. Humans harbor two functionally distinct NK-cell subsets exerting either predominantly cytotoxic (CD56(dim)CD16(+)) or immunoregulatory (CD56(bright)CD16(-)) functions. We analyzed these two subsets and their functions in the peripheral blood of untreated patients with relapsing-remitting MS compared with healthy blood donors. While ex vivo frequencies of CD56(bright)CD16(-) and CD56(dim)CD16(+) NK cells were similar in patients and controls, we found that cytokine-driven in vitro accumulation and IFN-γ production of CD56(bright)CD16(-) NK cells but not of their CD56(dim)CD16(+) counterparts were substantially diminished in MS. Impaired expansion of CD56(bright)CD16(-) NK cells was cell intrinsic because the observed effects could be reproduced with purified NK cells in an independent cohort of patients and controls. In contrast, cytolytic NK-cell activity toward the human erythromyeloblastoid leukemia cell line K562, the allogeneic CD4(+) T cell line CEM and allogeneic primary CD4(+) T-cell blasts was unchanged. Thus, characteristic functions of CD56(bright)CD16(-) NK cells, namely cytokine-induced NK cell expansion and IFN-γ production, are compromised in the NK cell compartment of MS patients.