NK cells possess both effector and regulatory activities that may be important during the antitumor immune response. In fact, the generation of antitumor immunity by the administration of an agonistic mAb against CD137 is NK cell-dependent. In this study, we report that NK cells could be induced by IL-2 and IL-15 to express CD137 and ligation of CD137-stimulated NK cell proliferation and IFN-γ secretion, but not their cytolytic activity. Importantly, CD137-stimulated NK cells promoted the expansion of activated T cells in vitro, demonstrating immunoregulatory or “helper” activity for CD8+ CTL. Furthermore, tumor-specific CTL activity against P815 tumor Ags was abrogated following anti-CD137 treatment in NK-depleted mice. We further demonstrate that CD137-stimulated helper NK cells expressed the high-affinity IL-2R and were hyperresponsive to IL-2. Taken together with previous findings that CD137 is a critical receptor for costimulation of T cells, our findings suggest that CD137 is a stimulatory receptor for NK cells involved in the crosstalk between innate and adaptive immunity.