The aim of this randomized open-label study was to compare a bimonthly with a monthly regimen of 5-fluorouracil (5-FU) and leucovorin for the adjuvant treatment of colon and high-rectum adenocarcinoma. The bimonthly regimen was administered for 2 consecutive days every 14 days as d,L-leucovorin 200 mg/m2 or L-leucovorin 100 mg/m2 as a 2-hour infusion followed by 5-FU bolus of 400 mg/m2 and a 600 mg/m2 5-FU 22-hour continuous infusion (LVSFU2). In the monthly regimen, d,L-leucovorin 200 mg/m2 or L-leucovorin 100 mg/m2 15-minute infusion followed by a 400 mg/m2 15 minute 5-FU bolus was administered for 5 consecutive days every 28 days (FUFOL). Nine hundred five patients with recently resected stage B2 or C colon or high-rectum adenocarcinoma (inferior pole of the tumor subperitoneal) were recruited into the study. Patients were randomized in a 2 x 2 factorial design to receive either LV5FU2 or FUFOL for 24 or 36 weeks. Characteristics of the patients in the two different treatment groups were similar at baseline. Compliance was good. Mean 5-FU dose intensities were 930 mg/ m2/wk and 463 mg/m2/wk for LVSFU2 and FUFOL, respectively. The incidence of maximal grade III-IV toxicities for LVSFU2 and FUFOL was neutropenia 6% and 16% (P < .001), diarrhea 4% and 10% (P < .001), and mucositis 2% and 7% (P < .001), respectively. Maximum grade III-IV toxicities in the LV5FU2 treatment group were significantly lower than in the FUFOL group (10% v 26%; P < .001). Although patients in the LV5FU2 group received twice the dose of 5-FU compared with those in the FUFOL group, LV5FU2 was shown to be less toxic. Efficacy data will be available in 2001.