Thirty patients with locally advanced nonresectable non-small cell bronchogenic carcinoma were studied for tolerance and response to 120-hour continuously infused 5-FU (IFU) combined with chest x-ray therapy. The IFU was escalated in patient groups from 20 to 35 mg/kg/24 hours. Radiation was also escalated in patient subsets from 200 to 400 rads/fraction using a split-course technique to a total dose of 5000 rads. Tolerance was exceeded by IFU doses of 35 mg/kg/day. The complete response rate was ten of 28 patients (36%); a complete response was associated with a significantly prolonged survival. Radiosensitization of lung cancer by IFU appears clinically feasible in man.