OBJECTIVE Colon cancer is one of the major health problems in industrialized countries, and its incidence appears to be increasing. Surgical resectability is the most important prognostic determinant, although despite apparently curative surgery, recurrent tumors are common. Metastatic disease cannot be cured, and thus, there is a need for better adjuvant therapies. METHODS Two hundred and thirty-nine patients with surgically resected colon cancer in Dukes' stage B2 or C were randomly assigned to chemotherapy or observation alone to determine whether adjuvant chemotherapy could effectively reduce the rate of cancer recurrence. One hundred and twenty-one patients in stage B2 and 118 patients in stage C were enrolled in the study. Adjuvant treatment consisted of folinic acid 200 mg/m2, intravenously, plus 5-fluorouracil 400 mg/m2, intravenously, on days 1-5 every 4 weeks for 12 cycles. RESULTS In stage B2, no significant difference between the adjuvant arm and the observation arm was noted. In stage C, adjuvant chemotherapy produced an advantage over observation in terms of a reduction in cancer recurrence rate with prolongation of a disease-free interval (P = 0.0016) and an improvement in overall survival (P = 0.0025). CONCLUSIONS This study shows that folinic acid plus 5-fluorouracil adjuvant chemotherapy is effective in patients with surgically resected Dukes' stage C colon carcinoma.