Histone deacetylases (HDAC) have been identified as therapeutic targets due to their regulatory function in DNA structure and organization. LBH589 is a novel inhibitor of class I and II HDACs. We studied the effect of LBH589 and ionizing radiation (IR) on DNA repair in two human non-small cell lung cancer (NSCLC) cell lines (H23 and H460). γ-H2AX foci present at DNA double-strand breaks (DSBs) were detected in the nuclei following 3 Gy irradiation for up to 6 hours. LBH589 administered before irradiation increased the duration of γ-H2AX foci beyond 24 hours. Furthermore, radiation alone induced translocation of HDAC4 to the nucleus. In contrast, treatment with LBH589 followed by irradiation resulted in HDAC4 confinement to the cytoplasm, indicating that HDAC inhibition affects the nuclear localization of HDAC4. The findings that LBH589 confines HDAC4 to the cytoplasm and increases the duration of γ-H2AX foci in irradiated cell lines suggest that HDAC4 participates in DNA damage signaling following IR. Annexin-propidium iodide flow cytometry assays, cell morphology studies, and cleaved caspase-3 Western blot analysis revealed a synergistic effect of LBH589 with IR in inducing apoptosis. Clonogenic survival showed a greater than additive effect when LBH589 was administered before irradiation compared with irradiation alone. In vivo tumor volume studies showed a growth delay of 20 days with combined treatment compared with 4 and 2 days for radiation or LBH589 alone. This study identifies HDAC4 as a biomarker of LBH589 activity and recognizes the ability of LBH589 to sensitize human NSCLC to radiation-induced DNA DSBs.