The radiosensitization properties of 5-FU are well documented, and clinical trials have suggested improved local control and survival in head and neck cancer. Clinical trials to date have used bolus injection or short term (≤5 days) 5-FU infusions. To determine the maximum tolerated dose (MTD) of 5-FU given as continuous intravenous infusion for 12 weeks concomitant with conventional radiation therapy, 18 patients with advanced inoperable head and neck cancers were treated with conventional irradiation and 100, 200, 250, or 300 mg/m2/day of 5-FU. A dose of 250 mg/m2/day was determined to be the maximum tolerated dose and is recommended for Phase 11 studies. © 1991.