Several studies have shown solid evidence for the potential value of targeting epidermal growth factor receptor (EGFR) signaling to enhance the antitumor activity of radiation. However, therapeutic resistance has emerged as an important clinical issue. Here, we investigated whether strategies for targeting EGFR-associated downstream signaling would radiosensitize a panel of non-small cell lung cancer cell lines. Inhibition of K-RAS using RNA interference attenuated downstream signaling and increased radiosensitivity of A549 and H460 cells, whereas inhibition of EGFR did not. A549 cells harboring a K-RAS mutation at codon V12 were radiosensitized by small interfering RNA (siRNA) targeting this codon. H460 cells having mutation at codon V61 was radiosensitized by siRNA targeting of this mutation. K-RAS siRNA did not radiosensitize H1299 cells possessing wild-type K-RAS. Inhibition of the phosphoinositide 3-kinase (PI3K)-AKT-mammalian target of rapamycin pathway led to significant radiosensitization of the two cell lines, whereas selective inhibition of extracellular signal-regulated kinase signaling did not. Inhibitors targeting the PI3K-AKT-mTOR pathway also abrogated G(2) arrest following irradiation and induced gammaH2AX foci formation. A dual inhibitor of class I PI3K and mammalian target of rapamycin effectively increased the radiosensitivity of A549 and H460 cells. Inhibition of PI3K-AKT signaling was associated with the downregulation of DNA-PKs. Although apoptosis was the primary mode of cell death when cells were pretreated with LY294002 or AKT inhibitor VIII, cells pretreated with rapamycin or PI-103 showed mixed modes of cell death, including apoptosis and autophagy. Our results suggest possible mechanisms for counteracting EGFR prosurvival signaling implicated in radioresistance and offer an alternative strategy for overcoming resistance to EGFR inhibitors used in combination with irradiation.