Hand–foot syndrome (HFS) is a common side-effect of capecitabine. S-1 is an oral fluoropyrimidine with comparable efficacy to capecitabine in gastrointestinal cancers but associated with a lower incidence of HFS in Asian patients. This study compares the incidence of HFS between S-1 and capecitabine as first-line treatment in Western metastatic colorectal cancer (mCRC) patients. Patients with previously untreated mCRC and planned treatment with fluoropyrimidine monochemotherapy were randomized 1 : 1 to receive either capecitabine (1250 mg/m2 orally for patients <70 years; 1000 mg/m2 for patients ≥70 years, twice daily on days 1–14) or S-1 (30 mg/m2 orally twice daily on days 1–14) in 3-weekly cycles, with bevacizumab optional in both groups. The primary endpoint was the incidence of any grade HFS, as assessed by both physicians and patients (diaries). Secondary endpoints included grade 3 HFS, other toxicities, relative dose intensity, progression-free survival, response rate and overall survival. A total of 161 patients were randomized in 27 centres. The incidence of any grade HFS as assessed by physicians was 73% in the capecitabine group (n = 80) and 45% in the S-1 group (n = 80) odds ratio (95% confidence interval) 0.31 (0.16–0.60),P = 0.0005. The incidence of grade 3 HFS was 21% and 4% (P = 0.003), respectively. Patient-assessed any grade HFS was 84% and 58%, respectively (P = 0.004). Grade 3 anorexia was more common in the S-1 group (3% versus 13%,P = 0.03). Median relative dose intensity was 88% in the capecitabine group and 95% in the S-1 group (P = 0.026). There were no statistically significant differences in median progression-free survival, response rate and overall survival rates. Treatment with S-1 in Western mCRC patients is associated with a significantly lower incidence of HFS compared with capecitabine, with comparable efficacy. NCT01918852.