In this study, a combination of the tumour-targeting antibody trastuzumab (which is specific for human epidermal growth factor receptor 2 (HER2)) and a natural killer (NK) cell-activating antibody specific for CD137 is shown to be highly effective in treating breast cancer. Trastuzumab leads to the elimination of HER2+ breast cancer cells mainly through antibody-dependent cell-mediated cytotoxicity by NK cells. NK cells exposed to trastuzumab-coated HER2+ tumour cells upregulate the co-stimulatory molecule CD137, and subsequent treatment with a CD137-specific agonistic antibody improved their ability to kill trastuzumab-coated HER2+ tumour cells in vitro. Moreover, athymic mice (which lack T cells but have normal NK cells) that were engrafted with human breast cancer cells and treated with trastuzumab followed by the CD137-specific antibody showed a marked reduction in tumour size and mortality compared with mice treated with only one antibody. The enhanced cytotoxicity was restricted to antibody-coated tumour cells, which suggests that this combined therapy could be applicable to other cancer-targeting antibodies.