PRIOR CONCURRENT THERAPY: Biologic therapy: No concurrent prophylactic use of growth factors except for documented febrile neutropenia or sepsis Chemotherapy: At least 12 months since prior fluorouracil based adjuvant or neoadjuvant chemotherapy No more than 1 prior regimen No prior chemotherapy for advanced metastatic colorectal cancer No other concurrent chemotherapy No concurrent investigational antineoplastic drugs Endocrine therapy: See Disease Characteristics Radiotherapy: At least 4 weeks since prior radiotherapy and recovered No prior radiotherapy to the only site of measurable disease No concurrent radiotherapy Surgery: Not specified Other: See Disease Characteristics At least 30 days since prior investigational drug