We randomized 224 patients with resected Dukes' stage B2 or C colorectal cancer to either an untreated control group or to a group receiving 7 days of fluorouracil therapy (500 mg/m2 per day) by portal vein infusion. Randomization was accomplished during surgery after staging by frozen section. Only 5 (2.2%) of our 224 patients were ineligible, but an additional 10 patients assigned to portal vein infusion could not be treated because of technical problems with catheter placement. Toxic reactions were mild. There was only 1 postoperative death on each study arm. At present, the median follow-up for all patients is 5.5 years (range, 1.5 to 9.5 years). Interval to progression and survival curves essentially overlap. The same lack of treatment effect is seen in both the stage B and C subsets.