Fluorouracil and Irinotecan With or Without Leucovorin Compared With Observation in Treating Patients Who Have Undergone Surgery for Stage II Colon Cancer

RATIONALE: Drugs used in chemotherapy, such as irinotecan, leucovorin, and fluorouracil, work in different ways to stop tumor cells from dividing so they stop growing or die. Combining more than one drug and giving them after surgery may kill any remaining tumor cells. It is not yet known whether combination chemotherapy is more effective than observation alone in treating patients who have undergone surgery for colon cancer. PURPOSE: This randomized phase III trial is studying irinotecan and fluorouracil with or without leucovorin to see how well they work compared to observation alone in treating patients who have undergone surgery for stage II colon cancer.