This study had two major goals: (1) to assess the effectiveness of a regimen of fluorouracil (5-FU) plus levamisole plus leucovorin as postoperative surgical adjuvant therapy for patients with high-risk colon cancer, and (2) to evaluate 6 months versus 12 months of chemotherapy. Patients with poor-prognosis stage II or III colon cancer were randomly assigned to receive adjuvant chemotherapy with either intensive-course 5-FU and leucovorin combined with levamisole, or a standard regimen of 5-FU plus levamisole. Patients were also randomly assigned to receive either 12 months or 6 months of chemotherapy, which resulted in four treatment groups. Eight hundred ninety-one of 915 patients entered (97.4%) were eligible. The median follow-up duration is 5.1 years for patients still alive. There was a difference among the four treatment groups with respect to patient survival, and a significant duration-by-regimen interaction was observed. Specifically, standard 5-FU plus levamisole was inferior to 5-FU plus leucovorin plus levamisole when treatment was given for 6 months (5-year survival rate, 60% v 70%; P < .01). There was no significant improvement in patient survival when chemotherapy was given for 12 months compared with 6 months. When chemotherapy was given for 6 months, standard 5-FU plus levamisole was associated with inferior patient survival compared with intensive-course 5-FU plus leucovorin plus levamisole. These data suggest that 5-FU plus levamisole for 6 months should not be used in clinical practice, whereas 6 months of treatment with 5-FU plus leucovorin plus levamisole is effective.