RATIONALE: Drugs used in chemotherapy use different ways to stop tumor cells from dividing so they stop growing or die. It is not yet known whether fluorouracil and leucovorin plus irinotecan is more effective than fluorouracil and leucovorin alone for colorectal cancer.

PURPOSE: Randomized phase III trial to compare the effectiveness of fluorouracil and leucovorin with or without irinotecan in treating patients who have metastatic colorectal cancer.

OBJECTIVES: I. Compare the efficacy and toxicity of high-dose fluorouracil and leucovorin calcium with or without irinotecan in patients with metastatic adenocarcinoma of the colon or rectum. II. Compare progression-free survival, overall survival, response rate, and duration of response in patients treated with these 2 regimens. III. Compare quality of life of patients treated with these 2 regimens.

OUTLINE: This is a randomized, multicenter study. Patients are randomized to one of two treatment arms. Arm I: Patients receive leucovorin calcium IV over 2 hours followed by fluorouracil IV over 24 hours on days 1, 8, 15, 22, 29, and 36. Arm II: Patients receive irinotecan IV over 30 minutes followed by leucovorin calcium IV over 2 hours and fluorouracil IV over 24 hours on days 1, 8, 15, 22, 29, and 36. Treatment in both arms repeats every 7 weeks in the absence of disease progression or unacceptable toxicity. Patients in arm I who develop disease progression begin second-line therapy comprising irinotecan, fluorouracil, and leucovorin calcium within 2 months of progression. Patients with complete response are taken off study after receiving treatment for one year. Quality of life is assessed before beginning study, after completion of each course, at 4 weeks after completion of study, and then every 2 months until disease progression or death. Patients are followed every 2 months until disease progression or death.

PROJECTED ACCRUAL: A total of 430 patients (215 per arm) will be accrued for this study within 2 years.

DISEASE CHARACTERISTICS: Histologically proven metastatic adenocarcinoma of the colon or rectum Measurable or evaluable disease outside of any prior radiation port No CNS metastases

PATIENT CHARACTERISTICS: Age: 18 and over Performance status: WHO 0-2 Life expectancy: Not specified Hematopoietic: WBC at least 3,000/mm3 Platelet count at least 100,000/mm3 Hepatic: Bilirubin no greater than 1.25 times upper limit of normal (ULN) (1.5 times ULN if liver metastasis present) AST and ALT no greater than 3 times ULN (5 times ULN if liver metastasis present) Renal: Creatinine no greater than 1.25 times ULN Cardiovascular: No severe cardiac disease No uncontrolled angina pectoris No myocardial infarction within the past 6 months Gastrointestinal: No unresolved bowel obstruction or subobstruction No uncontrolled Crohn's disease or ulcerative colitis No history of chronic diarrhea Other: No second malignancy except carcinoma in situ of the cervix or nonmelanomatous skin cancer No other uncontrolled severe medical condition Not pregnant or nursing Fertile patients must use effective contraception

PRIOR CONCURRENT THERAPY: Biologic therapy: Not specified Chemotherapy: No prior chemotherapy for metastatic disease No prior adjuvant chemotherapy containing topoisomerase I inhibitors At least 6 months since other prior adjuvant chemotherapy Endocrine therapy: Concurrent corticosteroids allowed Radiotherapy: See Disease Characteristics Surgery: Not specified Other: At least 4 weeks since other prior investigational drugs No other concurrent anticancer therapy