BACKGROUND Adjuvant chemotherapy for colon cancer has been established during the past decade. From 1990 until recently treatment with 5- fluorouracil (5-FU) and levamisole (LEV) lasting 12 months was recommended as standard treatment. At the initiation of this study in 1993 improvement of adjuvant therapy was expected by the modulation of 5-FU with folinic acid (FA). Therefore, we decided to perform a prospective randomized multicenter trial to compare standard 5-FU/LEV to 5-FU/FA for either 6 or 12 months. PATIENTS AND METHODS Patients with stage III colon cancer after curative en bloc resection were randomized in 3 treatment groups: arm A (5-FU/LEV, weekly, 12 months), arm B (5-FU/FA, days 1-5, every 4 weeks, 12 months) and arm C (like B, 6 months). RESULTS Between March 1993 and November 1997, 180 patients were randomized into the study, 155 were eligible for further evaluation. The interim analysis in November 2000 showed no significant difference for recurrence and disease-free survival in arm B and C, therefore the data from both 5-FU/FA treatment arms (B+C) were combined for comparison with 5-FU/LEV-treatment (A). Most pronounced toxicity in all treatment arms was mild nausea, loss of appetite and leukopenia. A tendency for more diarrhea and stomatitis was observed in arm B+C. After a median follow-up of 36.2 months no significant difference was seen for disease free survival (p = 0.9) and overall survival (p = 1.0). 3-year recurrence rates were 39.6% in arm A and 39.1% in arm B+C, 3-year survival rates amounted to 74.1% in arm A and 74.9% in arm B+C. CONCLUSION Only a limited number of patients could be recruited in this study. The observed data support the results of other studies, which concluded that 6 months (or 12 months) treatment with 5-FU/FA is equivalent to 12 months treatment with 5-FU/LEV. Therefore the 6 months treatment with 5-FU/FA can be supported as standard for adjuvant therapy of stage III colon cancer.