ZSTK474 is a novel orally applicable phosphoinositide 3-kinase-specific inhibitor that strongly inhibits cancer cell proliferation. To further explore the antitumor effect of ZSTK474 for future clinical usage, we studied its combined effects with radiation. The proliferation of HeLa cells was inhibited by treatment with X-rays alone or ZSTK474 alone. Combination treatment using X-rays then ZSTK474 given orally for 8 days, starting 24 h post-irradiation, significantly enhanced cell growth inhibition. The combined effect was also observed for clonogenic survival with continuous ZSTK474 treatment. Western blot analysis showed enhanced phosphorylation of Akt and GSK-3β by X-irradiation, whereas phosphorylation was inhibited by ZSTK474 treatment alone. Treatment with ZSTK474 after X-irradiation also inhibited phosphorylation, and remarkably inhibited xenograft tumor growth. Combined treatment with X-rays and ZSTK474 has greater therapeutic potential than radiation or drug therapy alone, both in vitro and in vivo. (Cancer Sci 2011; 102: 1176–1180)