S-1 (Teysuno®) is an oral fluoropyrimidine that has shown comparable efficacy to 5FU and capecitabine in gastrointestinal cancers but is associated with a much lower incidence of HFS. Studies on S-1 have mainly been performed in Asian patients,which population has known differences in tumour biology and toxicity compared to Western population. S-1 has shown comparable efficacy to other fluoropyrimidines as monotherapy or in combination chemotherapy schedules in several gastrointestinal tumors. However, given the lack of data from prospective studies on S-1 as monochemotherapy in metastatic colorectal cancer in Western patients, this study is designed to compare S-1 and capecitabine monotherapy in terms of safety, with particular interest in HFS, in metastatic colorectal cancer patients.