Cancer stem cells (CSCs) are controversial and reported to be resistant to cytotoxic therapies such as radiation therapy (RT) and chemotherapy and responsible for cancer recurrences. They are defined by their ability to initiate tumors in vivo. A recent series of high impact studies have validated the critical importance of these cells. Previous studies demonstrate that natural killer (NK) cells are able to detect and reject allogeneic hematopoietic stem cells. We hypothesized that CSCs should be susceptible to NK immune attack and this susceptibility would be enhanced by the immunomodulatory effects of RT. All studies were approved by the UC Davis IRB and IACUC. Human cancer cells were xenografted into NOD-SCID IL2 receptor gamma chain null (NSG) mice. Primary human tumor tissue was obtained fresh from the UC Davis Cancer Center Biorepository. Human NK cells were isolated from leukocyte filters obtained from healthy donors. Localized RT was delivered using a clinical grade linear accelerator. Expression of CSC markers including aldehyde dehydrogenase (ALDH), CD133, CD24, and CD44 were analyzed by FACS using a BD Fortessa flow cytometer. Data were analyzed with FlowJo software version 7.2. Parametric and non-parametric statistical tests were performed as appropriate. We demonstrated that sorted CSCs were able to form tumors in vivo whereas non CSCs were very poorly tumorigenic. Coincubation with activated NK cells significantly reduced CSCs in cell lines (sarcoma 5±1% to 0.3±0.1%, breast 26±1.5 to 4±0.5%, pancreas 20±2% to 11±1.5%, P<0.05) and primary human tumors (liposarcoma 39% to 24%, pancreatic cancer 58% to 35%, P 120 days. This radio-immunotherapy regimen resulted in a synergistic decrease in CSC populations and enhanced survival in xenogeneic tumor models. These data suggest that NK cells can be used clinically to treat solid tumors and may provide the greatest benefit when used in combination with RT by targeting the population of CSCs that remain after RT has debulked the tumor of non-CSCs and increased the immunogenicity of the remaining CSC population. These finding could have profound consequences for recurrences after RT and provide additional evidence that combining RT and immunotherapy can lead to synergistic anti-tumor effects.