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

DISEASE CHARACTERISTICS: Histologically confirmed adenocarcinoma of the colon Stage II disease (pT3, N0 or pT4, N0) Penetration of the subserosa or serosa No lymph node metastases At least 12 lymph nodes analyzed More than 1 synchronous primary colon tumor allowed Staging determined for the more advanced tumor Curative radical resection within the past 2-8 weeks required Proximal, distal, and radical margins must be free of tumor (R0 resection) No rectal tumors Gross distal margin of the primary tumor must lie above the peritoneal reflection No known familial adenomatous polyposis No hereditary nonpolyposis colorectal cancer No distant metastases PATIENT CHARACTERISTICS: Age 18 to 75 Performance status WHO 0-1 Life expectancy Not specified Hematopoietic Granulocyte count ≥ 2,000/mm^3 Platelet count ≥ 100,000mm^3 Hemoglobin ≥ 10 g/dL Hepatic Bilirubin < 1.25 times upper limit of normal (ULN) No known Gilbert's syndrome Renal Creatinine < 1.25 times ULN Cardiovascular No severe or uncontrolled coronary disease No severe heart failure No uncontrolled arterial hypertension No myocardial infarction within the past year No cerebral vascular accident within the past year Cardiac arrhythmia allowed provided patient is on proper anticoagulation therapy* NOTE: *Aspirin is not considered proper anticoagulation Gastrointestinal No Gardner's syndrome No Turcot's syndrome No Crohn's disease No ulcerative colitis Other No other prior malignancy except adequately treated basal cell skin cancer or carcinoma in situ of the cervix No other serious disease No contraindication to any study drugs No known allergy to leucovorin calcium Not pregnant or nursing Negative pregnancy test Fertile patients must use effective contraception during and for up to 6 months after study treatment PRIOR CONCURRENT THERAPY: Biologic therapy Not specified Chemotherapy No prior chemotherapy Endocrine therapy Not specified Radiotherapy No prior radiotherapy Surgery See Disease Characteristics Other No other concurrent anticancer therapy No concurrent vitamin supplements containing folic acid