Chemoradiotherapy is a useful treatment strategy in patients with locally advanced cancers. In particular, combination of 5-fluorouracil (5-FU) with X-ray irradiation is effective for the treatment of some types of gastrointestinal cancers. We investigated the antitumor effects of combination treatment with X-ray and S-1, a unique formulation of 5-FU, on human cancer xenografts in nude mice and compared the efficacy of this treatment to that of radiotherapy combined with cisplatin, UFT, another oral 5-FU prodrug, and intravenous 5-FU. Tumors implanted into the left hind legs of mice were treated with a dose of 2 or 5 Gy X-ray irradiation on days 1 and 8, and S-1, UFT and 5-FU were administered for 14 days. The efficacy of combined treatment with 8.3 mg/kg S-1 and 2 Gy X-ray irradiation in treating non-small cell lung cancer xenografts (Lu-99 and LC-11) was significantly higher than that of treatment with S-1 alone or 2 Gy X-ray irradiation alone, and the antitumor activity of combined treatment was similar to that of 5 Gy X-ray irradiation alone. Although 8.3 mg/kg S-1 and 17.5 mg/kg UFT had equivalent antitumor activity; the antitumor efficacy of combination treatment with S-1 and 2 Gy X-ray irradiation on LC-11 tumors was significantly higher than that of combination treatment with UFT and 2 Gy X-ray irradiation. Combination treatment with S-1 and X-ray irradiation was also more effective against pancreatic tumors than combination treatment with intravenous 5-FU and X-ray irradiation. To elucidate the reason for the increased antitumor efficacy of combination treatment with S-1 and X-ray irradiation, the antitumor effect of gimeracil, one of the components of S-1, was tested in combination with 2 Gy X-ray irradiation. These experiments demonstrated that gimeracil enhanced the efficacy of X-ray irradiation against lung as well as head and neck cancer xenografts in a dose-dependent manner. Furthermore, we observed decreased expression of γ-H2AX protein, a marker of DNA repair, in LC-11 tumors treated with X-ray irradiation and gimeracil compared to that observed in tumors treated with X-ray irradiation alone, suggesting that gimeracil may inhibit rapid repair of X-ray-induced DNA damage in tumors. The present study suggests that chemoradiotherapy using S-1 acts through a novel mechanism and may prove useful in treating patients with locally advanced cancers whose disease progression is difficult to control using chemotherapy alone.