Overexpression of the Aurora-B kinase correlates with oncogenic transformation and poor prognosis. We evaluated the effects of the bona fide Aurora-B kinase inhibitor AZD1152 on tumor responses to ionizing radiation (IR). When p53(wt) HCT116 and A549 cells were pretreated with AZD1152-HQPA prior to IR, additive effects were observed. Interestingly, more pronounced tumoricidal effects were observed in p53-deficient HCT116 and HT29 cells, as well as A549 cells treated with the p53 inhibitor cyclic pifithrin-alpha. In vivo studies on xenografted mice confirmed enhanced tumor growth delay after the combination of IR plus AZD1152-IR as compared to IR alone. Again, this effect was more pronounced with p53-/- HCT116 and p53-mutant xenografts. The AZD1152-mediated radiosensitization was mimicked by knockdown of Aurora-B with a short interference RNA or by inhibition of Aurora-B by transfection with an inducible kinase-dead Aurora-B. The radiosensitizing effect of AZD1152 was lost in CHK2-/- and 14-3-3-/- HCT116 cells. Altogether, these data indicate that AZD1152 can radiosensitize tumor cell lines in vitro and in vivo, the fact that these effects are exacerbated in p53-deficient cancer cells is of potential interest for further clinical development.