BACKGROUND: Donor alloreactive natural killer (NK) cells have a potent antileukemic effect in haploidentical stem cell transplantation. Whether alloreactive NK cells are able to specifically kill fresh tumor cells from primary solid tumors was analyzed. METHODS: NK cells were purified from healthy donors for the expression of inhibitory killer cell immunoglobulin (Ig)-like receptors (KIRs), ex vivo expanded, and used as effector cells. Their cytotoxic effect on tumor cells freshly obtained from surgical specimens was assessed by means of a single-cell cytotoxic assay (SCCA). RESULTS: Tumor cells from 1 ovarian, 1 gastric, 3 colon, and 4 renal cell cancers were analyzed and found susceptible to alloreactive NK cell killing (>20% lysis at an effector cell to target cell [E:T] ratio of 10:1 for tumor cells not expressing at least 1 human lymphocyte antigen [HLA] class I KIR-ligand group). Remarkably, NK cells that recognized specific HLA-C group mismatches were able to kill HLA-C KIR ligand-mismatched tumor cells, whereas no lysis of target cells occurred with KIR ligand-matched tumor targets. CONCLUSIONS: Alloreactive NK-cell mediated antitumor effects might provide useful insights for designing new cell therapy approaches against solid tumors.