RATIONALE: Drugs used in chemotherapy use different ways to stop tumor cells from dividing so they stop growing or die. Combining more than one drug is a way to kill more tumor cells.

PURPOSE: Randomized phase III trial to determine the effectiveness of uracil, tegafur,and leucovorin compared with fluorouracil plus leucovorin in patients with resected stage II or stage III colon cancer.

OBJECTIVES: I. Compare the relative efficacy of oral uracil/tegafur (UFT) with leucovorin (CF) vs. fluorouracil (5-FU) with CF in prolonging overall and disease-free survival in patients with potentially curatively resected stage II/III adenocarcinoma of the colon. II. Compare the prognostic significance of several biomarkers alone or in combination, including DNA mismatch repair gene mutations, p53, deleted colon cancer gene, proliferation status, and thymidylate synthase, in patients treated with UFT/CF vs. 5-FU/CF. III. Evaluate the relationships of various biomarkers to each other and their association with patient and tumor characteristics. IV. Compare quality of life in patients treated with UFT/CF vs. 5-FU/CF.

OUTLINE: This is a randomized study. Patients are stratified by number of positive lymph nodes and participating institution. For the quality-of-life portion of the study, patients are stratified by age, sex, and ethnicity. Treatment begins within 6 weeks after curative resection and within 1 week of randomization. Patients are randomly assigned to one of two groups. The first group receives intravenous leucovorin followed by intravenous fluorouracil weekly for 6 weeks. Treatment repeats every 8 weeks for a total of 3 courses. The second group receives oral uracil/tegafur, and oral leucovorin every 8 hours for 28 days. Treatment repeats every 5 weeks for a total of 5 courses. No concurrent halogenated antiviral agents (e.g., sorivudine) are permitted. After completing treatment, patients complete quality-of-life questionnaires at one year.

PROJECTED ACCRUAL: Approximately 1,500 patients will be entered over 3 years to provide 1,452 evaluable patients.

DISEASE CHARACTERISTICS: Histologically confirmed adenocarcinoma of the colon that has been curatively resected within 42 days prior to entry, as follows: Stage II (T3-4 N0 M0, Modified Astler-Coller B2/3), i.e., tumor invasion of the wall of the colon or extension into the pericolonic tissue Stage III (any T N1-3 M0), i.e., tumor invasion of any depth No sarcoma, lymphoma, or carcinoid histology No laparoscopically assisted colectomy unless performed on protocol NCCTG-934653 (intergroup study INT-0146) Entire tumor located above the peritoneal reflection on surgical exploration or more than 15 cm from anal verge on endoscopy Involved adjacent structures (e.g., bladder, small intestine, ovary) removed en bloc with histologically negative margins More than 1 synchronous primary colon tumor allowed Most advanced tumor used for stage assignment No previous or synchronous rectal cancer Intestinal obstruction allowed Preliminary or complementary colostomy allowed Walled-off perforation allowed No free perforation, i.e., free air or fluid in abdomen No prior invasive colon or rectal malignancy regardless of disease-free interval

PATIENT CHARACTERISTICS: Age: Not specified Performance status: ECOG 0-2 Life expectancy: At least 10 years (excluding cancer diagnosis) Hematopoietic: WBC at least 4,000/mm3 Platelet count at least 100,000/mm3 Hepatic: Bilirubin normal AST or ALT normal Renal: Creatinine normal Cardiovascular: No active ischemic heart disease No NYHA class III/IV status No myocardial infarction within 6 months No symptomatic arrhythmia within 6 months Other: No nonmalignant systemic disease that precludes protocol treatment No psychiatric or addictive disorder that precludes informed consent No second malignancy within 10 years except: Effectively treated nonmelanomatous skin cancer Surgically cured carcinoma in situ of the cervix Lobular carcinoma in situ of the breast No pregnant women Effective contraception required of fertile patients

PRIOR CONCURRENT THERAPY: See Disease Characteristics No prior chemotherapy or radiotherapy