The purpose of this trial was to examine the efficacy of the addition of levamisole (LEV) or interferon alfa (IFN) to an adjuvant chemotherapy with 5-fluorouracil (5-FU) in patients with stage III colon cancer. According to a 2 x 2 factorial study design, 598 patients were randomly assigned to one of four adjuvant treatment arms. Patients in arm one received 5-FU weekly for 1 year, patients in arm two 5-FU plus LEV, in arm three 5-FU plus IFN and patients in arm four 5-FU, LEV and IFN. The relative risk of relapse and the relative risk of death were significantly higher for patients treated with LEV compared with those without LEV treatment (HR 1.452, 95% CI 1.135-1.856, P=0.0028; HR 1.506, 95% CI 1.150-1.973, P=0.0027, respectively). No significant impact on survival was observed for therapy with IFN in the univariate analysis. The addition of LEV to adjuvant 5-FU significantly worsened the prognosis of patients with stage III colon cancer. Interferon alfa had no significant influence on survival when combined with adjuvant 5-FU, but increased the toxicity of therapy substantially.