DISEASE CHARACTERISTICS: Histologically or cytologically proven metastatic colorectal cancer Prior surgery required No prior treatment for metastatic disease Disease progression while on fluorouracil adjuvant therapy Bidimensionally measurable disease

PATIENT CHARACTERISTICS: Age: Over 18 Performance status: SWOG 0-2 Life expectancy: Not specified Hematopoietic: Platelet count at least 100,000/mm3 Absolute granulocyte count at least 1,500/mm3 Hepatic: Bilirubin no greater than 1.5 times upper limit of normal (ULN) SGOT or SGPT no greater than 3 times ULN (no greater than 5 times ULN if liver involvement) Renal: Creatinine clearance at least 50 mL/min Other: Must be able to swallow and retain oral medications No nutritional support by gastrostomy feeding tube or parenteral support No intractable vomiting or nausea Patients with history of prior malignancy must be currently disease free and off all therapy for that malignancy Not pregnant or nursing Fertile patients must use effective contraception

PRIOR CONCURRENT THERAPY: Biologic therapy: No concurrent immunotherapy Chemotherapy: See Disease Characteristics At least 3 weeks since prior chemotherapy and recovered No concurrent flucytosine No other concurrent chemotherapy Endocrine therapy: No concurrent hormonal therapy Radiotherapy: Prior adjuvant pelvic radiation or radiation to one area of recurrence allowed At least 4 weeks since prior radiotherapy and recovered Total area of bone marrow irradiation must not exceed 25% No concurrent radiotherapy Surgery: See Disease Characteristics At least 2 weeks since prior surgery for colorectal cancer and recovered No prior resection of the stomach or small intestine Other: No other concurrent anticancer therapy