We conducted a phase I/II trial of 5-fluorouracil (5-FU)/folinic acid (FA) and alpha-2b interferon (IFN) in 43 previously untreated patients with measurable metastatic colorectal cancer. Patients had disease progression prior to therapy consistent of 5-FU 500 mg/m2 (level A) or 600 mg/m2 (level B) as starting dose administered as a 2-hour infusion, FA 200 mg/m2, and alpha-2b IFN 5MU/m2 subcutaneously. All drugs were given on days 1 to 5 and cycles were repeated after 3 to 4 weeks. The aim of the study was to define the maximal tolerable dose (MTD) of 5-FU for this schedule. In the absence of toxicity above MTD, defined as diarrhea and mucositis of World Health Organization grade 2 and leukopenia of World Health Organization, grade 3 5-FU was increased in increments of 100 mg/m2 for further cycles. Twenty-four patients were started on level A; 18 were started on level B. Forty-two patients were evaluable for toxicity, 32 for response. Three of 32 patients achieved a partial response; in 22 of 32 patients, tumor stabilization occurred. Forty percent of patients started on level A developed grade 2 diarrhea; 28% of patients developed grade 2 or 3 mucositis. Of 18 patients on level B, two patients experienced grade 4 mucositis (11%) and grade 3 or 4 diarrhea (11%). One drug-related death in the presence of sepsis occurred. Due to 11% of patients with grade 4 toxicity when started on 600 mg/m2 5-FU and 40% of patients with grade 2 diarrhea when started on level A, MTD as starting dose for 5-FU is 500 mg/m2 as a 2-hour infusion when used in combination with FA and IFN on a day 1 to 5 active schedule. We observed a wide range of 5-FU dose tolerated by individual patients. While some patients experienced severe, mainly gastrointestinal, toxicity on lower levels of 5-FU, others tolerated much higher 5-FU doses (11 patients, 700 mg/m2; 5 patients, 800 mg/m2; and one patient, 900 mg/m2). Our data suggest that double modulation of 5-FU by FA and IFN may not be superior to modulation of 5-FU with either drug alone.