The role of antigen-specific CD3(+)CD8(+) cytotoxic T cells in the control of primary Epstein-Barr Virus (EBV) infection is well established. However, time is required for the antigen-specific immune response to develop and expand. In contrast, innate immune responses, such as natural killer (NK) cells, are considered vital early in the infection process. We analysed the scale, phenotype and function of the NK cell response during symptomatic primary EBV infection, infectious mononucleosis (IM) and showed that NK cell numbers were significantly elevated both at diagnosis of IM and in the first month following diagnosis. There were also significant changes in cell phenotype and function, an increase in the proportion of CD56(bright) cells at diagnosis, and freshly isolated cells showing an enhanced ability to kill EBV-infected cell lines. Moreover, in our cohort of IM patients higher NK cell counts were associated with significantly lower viral load in peripheral blood. Our results suggest NK cells have an important role in the control of primary EBV infection by eliminating infected B cells and augmenting the antigen-specific T cell response via release of immunomodulatory cytokines. The magnitude of the NK cell response may ultimately determine whether primary EBV infection has a clinical outcome.