To determine the maximum tolerated dose of 5-fluorouracil administered as a 120-hour continuous intravenous infusion to pediatric patients, we performed a phase I study using a starting dosage of 900 mg/m2/day. The maximum tolerated dosage (MTD) was 1,100 mg/m2/day. At this dosage level 40% of courses were complicated by grade 3 mucositis. Three additional patients were treated at the dosage level of 1,000 mg/m2/day after the MTD was determined. We recommend the dosage level of 1,000 mg/m2/day for phase II studies of 5-fluorouracil administered as a 120-hour continuous intravenous infusion to pediatric patients.