Advanced colorectal cancer can be effectively treated with S-1, as well as with a combination of oxaliplatin, 5-fluorouracil (5FU) and leucovorin (LV). However, S-1 together with oxaliplatin may provide a more convenient alternative to 5FU/LV. To evaluate the performance of S-1 combined with oxaliplatin for patients with colorectal cancer, we conducted a Phase I clinical trial in an outpatient setting. We administered S-1 to 15 patients with advanced colorectal adenocarcinoma for two weeks followed by one week of rest. Oxaliplatin was also administered on day 1 of the S-1 cycle. The dose of oxaliplatin was increased from 40 to 85 mg/m(2) to define the maximum tolerated dose and recommended dose in preparation for a Phase II trial. We administered 102 courses of treatment to 15 patients. Grade 3 thrombocytopenia developed in only 1 patient at a dose of 85 mg/m(2) of oxaliplatin plus oral S-1. No other grade 3-4 toxicities developed. No dose-limiting toxicity developed at level 4 of our regimen (oxaliplatin 85 mg/m(2)), and the recommended dose for a Phase II trial was 85 mg/m(2) of oxaliplatin in an outpatient setting.