BACKGROUND Capecitabine and oxaliplatin are both active anticancer agents in the treatment of patients with advanced colorectal cancer. The aim of this phase II study is to determine the efficacy and tolerability of combining oxaliplatin with capecitabine in the treatment of advanced non-pretreated colorectal cancer. METHODS Forty-three chemotherapy-naïve patients were enrolled. Capecitabine 2500 mg/m(2)/day was administered orally twice a day continuously for 14 days and oxaliplatin 120 mg/m(2) was administered as a 2-h infusion on day 1, repeated every 3 weeks. RESULTS Forty-three patients were assessable for toxicity and 39 for clinical activity: the main toxicity was grade 3 or 4 diarrhea, which occurred in 28% of the patients. The response rates were 44% [95% confidence interval (CI), 29.3% to 59.0%] and 48.7% (95% CI 33.0% to 64.4%) (intention-to-treat and per protocol analysis, respectively). The median overall survival was 20 months (95% CI 12-28). CONCLUSIONS Combining capecitabine and oxaliplatin yields promising activity in advanced colorectal cancer; therefore, the capecitabine dose we utilized is probably too high. The main toxicity is diarrhea, which is manageable with appropriate dose reductions. This combination may be preferable compared to a standard combination with infusional fluorouracil/leucovorin as it is more convenient and practical with similar efficacy. Thus, phase III trials are needed to clarify its role in the treatment of chemotherapy-naïve advanced colorectal cancer patients.