PURPOSE: There are few therapeutic options in patients with colorectal cancer that have progressed or recurred following 5-fluorouracil (5-FU) based therapy. We evaluated the efficacy and toxicity of oxaliplatin, 5-FU, leucovorin (Mayo clinic regimen) in 5-FU pretreated advanced colorectal cancer patients. MATERIALS AND METGODS: Twenty-eight patients were enrolled in this study between January 1999 and May 2001. Patients were treated with oxaliplatin 150 mg/m2 on day 1 as a 2-hr infusion and 5-FU 425 mg/m2, leucovorin 20 mg/m2, bolus for 5 days. Treatment courses were repeated in 4-week intervals. RESULTS: The objective response rate was 25% for 28 assessable patients, all cases registered a partial response. Eleven patients (39%) demonstrated stable disease, and ten (36%) progressed. The median response duration was 5.5 months, and the median time to progression was 6.3 months. The median overall survival time was 13.5 months from the start of the chemotherapy. From the 120 cycles analyzed, grade 3,4 hematologic toxicities included neutropenia: 1.6%, and thrombocytopenia: 1.6%. The frequent grade 3.4 non-hematologic adverse reactions were nausea/vomiting (25.0%), diarrhea (14.3%), stomititis (3.6%), and neuropathy (3.6%). There were no treatment-related deaths. CONCLUSION: This phase II study had relatively higher toxicity than previous studies, and did not show an increased significant response rate. These high levels of toxicity suggest that the study treatment combination of oxaliplatin with a full dose Mayo clinic regimen arm is no feasible. Therefore, this regimen will be discontinued and a safer regimen will be adopted.