A phase II study of S-1 plus leucovorin (LV) given in a 4-week schedule (2 weeks' administration followed by 2 weeks' rest) for patients with untreated metastatic colorectal cancer (mCRC) showed that the combination was effective, but grade 3 toxicities (diarrhea, stomatitis and anorexia) occurred at a relatively high rate. In this phase II study, we evaluated the efficacy and safety of a 2-week schedule of S-1 plus LV. Patients with mCRC received oral S-1 (40-60 mg) and LV (25 mg) twice daily for 1 week, followed by 1 week's rest. Treatment was repeated until disease progression or unacceptable toxicity. The primary endpoint was response rate. The pharmacokinetics of S-1 and LV in Chinese patients were evaluated on day 1 of the first cycle. Seventy-three patients were enrolled in Japan and China. Of 71 eligible patients, the response rate was 53.5%, and the disease control rate was 83.1%. Median progression-free survival and median overall survival were 6.5 and 24.3 months, respectively. The incidences of grade 3 toxicities were diarrhea 8.3%, stomatitis 8.3%, anorexia 2.8% and neutropenia 9.7%. There were no treatment-related deaths. The pharmacokinetics profiles of S-1 plus LV in Chinese patients were similar to those in Japanese patients. This 2-week schedule of S-1 plus LV showed good efficacy and better tolerability than the 4-week schedule. This therapy will be the base regimen for mCRC to be added by other cytotoxic or molecular-targeted drugs. The optimized treatment schedule for S-1 plus LV was 1 week on and 1 week off.