NK cells offer a first line of defense against viruses and are considered beneficial to the host during infection. Nevertheless, little is understood regarding the phenotype and function of NK cells in the lung during influenza virus infection. We found that the frequency of NK cells in mouse lung increased during influenza infection, with the majority of a mature phenotype. Cell surface CD107a and intracellular IFN‐γ were detected in cells expressing multiple NK‐cell receptors in infected lung, suggesting that NK cells were activated during infection. The activating receptor NKp46 was predominantly negative on such cells, possibly as a result of encountering influenza HA. Depletion of NK cells in vivo with anti‐asialo GM1 or anti‐NK1.1 reduced mortality from influenza infection and surviving mice recovered their body weight. Pathology induced by NK cells was only observed with high, not medium or low‐dose influenza infection, indicating that the severity of infection influences NK‐cell‐mediated pathology. Furthermore, adoptive transfer of NK cells from influenza‐infected lung, but not uninfected lung, resulted in more rapid weight loss and increased mortality of influenza‐infected mice. Our results indicate that during severe influenza infection of the lung, NK cells have a deleterious impact on the host, promoting mortality.