Natural killer (NK) cell clearance of tumor cell emboli following surgery is thought to be vital in preventing postoperative metastases. Using a mouse model of surgical stress, we transferred surgically stressed NK cells into NK-deficient mice and observed enhanced lung metastases in tumor-bearing mice as compared with mice that received untreated NK cells. These results establish that NK cells play a crucial role in mediating tumor clearance following surgery. Surgery markedly reduced NK cell total numbers in the spleen and affected NK cell migration. Ex vivo and in vivo tumor cell killing by NK cells were significantly reduced in surgically stressed mice. Furthermore, secreted tissue signals and myeloid-derived suppressor cell populations were altered in surgically stressed mice. Significantly, perioperative administration of oncolytic parapoxvirus ovis (ORFV) and vaccinia virus can reverse NK cell suppression, which correlates with a reduction in the postoperative formation of metastases. In human studies, postoperative cancer surgery patients had reduced NK cell cytotoxicity, and we show for the first time that oncolytic vaccinia virus markedly increases NK cell activity in patients with cancer. These data provide direct in vivo evidence that surgical stress impairs global NK cell function. Perioperative therapies aimed at enhancing NK cell function will reduce metastatic recurrence and improve survival in surgical cancer patients. Cancer Res; 73(1); 97–107. ©2012 AACR.