In this adjuvant three-arm multicenter trial, we studied whether modulating the standard 5-fluorouracil (5-FU) treatment with either folinic acid (FA) or interferon-alpha-2a (IFN-alpha) was superior to the recommended standard of adjuvant treatment in R0 resected colon cancer, 5-FU plus levamisole (LEV) for 12 months, in terms of toxicity and outcome. From July 1992 to October 1999, a total of 813 patients with resected colon cancer in stage II (T4N0M0; n = 63) or stage III (TxN1-3M0; n = 750) were randomized into three treatment groups and stratified according to N stage and participating centers (64 hospitals). The patients received a postoperative loading dose of 5-FU (450 mg/m2 on days 1 to 5 [arms A and C]) or 5-FU (450 mg/m2) plus FA (Rescuvolin, Medac, Hamburg, Germany, 200 mg/m2 on days 1 to 5 [arm B]). After completion of the first chemotherapy cycle, LEV was administered orally at a dosage of 150 mg per day on days 1 to 3, once every 2 weeks. After a 4-week chemotherapy-free interval, the treatment was continued weekly for 52 weeks. Treatment in one arm A ("standard") (n = 279) consisted of 5-FU intravenously (450 mg/m2 on day 1, once a week) plus LEV. 5-FU plus LEV was modulated in arm B (n = 283) with FA (200 mg/m2 on day 1, once a week) and in arm C (n = 251) with IFN-alpha at 6 million units three times a week repeated weekly. Treatment dosages were adjusted if toxic events above WHO grade 2 occurred. Patients were closely followed to determine recurrence and survival; the latter was calculated according to Kaplan-Meier analysis. Toxic events above WHO grade 2, mainly leukopenia, diarrhea, and nausea, occurred in 113 (14%) of 649 patients who had completed treatment in arms A (8.4%), B (13.5%), and C (31.7%). Discontinuance rates were as follows: 28% for all patients, 29% in arm A, 21% in arm B, and 34% in arm C. Overall relapse rates were 27% for all patients, 30% in arm A, 24% in arm B, and 28% in arm C. Relapses were local (8%), distant (78%), or combined (12%). Four-year overall survival rates in arms A, B, and C were 66.1%, 77.5%, and 66.2%, respectively. The 4-year survival rate in arm B was significantly higher compared to arm A (P <0.02, log-rank test) with arm A being equal to arm C. Adjuvant therapy with 5-FU plus FA plus LEV for 12 months is superior to the recommended standard (5-FU + LEV for 12 months). IFN-alpha modulation of 5-FU (plus LEV) adds to the toxicity with no therapeutic benefit.