This is an open-label, nonrandomized phase I trial to determine the safety and maximum tolerated dose of irinotecan with a fixed dose of UFT plus oral leucovorin in patients with advanced or metastatic colorectal cancer. A secondary objective of the study is to determine the response rate in this patient population. Adult patients with histologically confirmed advanced or metastatic colorectal carcinoma and no prior chemotherapy for advanced or metastatic disease, or those patients who have received adjuvant chemotherapy (> 6 months prior to the study entry) will be eligible to participate. All patients must have measurable or evaluable lesions. Symptoms will be evaluated at baseline and every 6 weeks thereafter. Computed tomography scans will be performed at baseline and every three cycles of treatment to assess tumor response. A total of six cycles of treatment may be given, depending on patient tolerance. Patients will be followed for a maximum of 12 months for time to progression, following the last dose of the study drug. Tumor response will be defined using standard World Health Organization criteria.