BACKGROUND The Adjuvant Chemotherapy Trial of S-1 for Gastric Cancer established oral S-1 administration for 1 year as the standard postoperative adjuvant chemotherapy for gastric cancer in Japan. We conducted a multicenter cooperative prospective study comparing daily and alternate-day S-1 administration as postoperative adjuvant therapy for gastric cancer. METHODS Patients with Stage II or III gastric cancer who underwent curative surgery were randomly assigned to receive standard daily S-1 administration [group A: 80-120 mg/day S-1 depending on body surface area (BSA); days 1-28 every 6 weeks for 1 year] or alternate-day administration (group B: 80-120 mg/day S-1 depending on BSA; alternate days for 15 months). Treatment completion rate was the primary endpoint, and relative dose intensity and safety, overall survival, and relapse-free survival (RFS) were secondary endpoints. RESULTS Seventy-three patients were enrolled. The treatment completion rate was 72.2 % in group A and 91.8 % in group B; the relative dose intensity was 67.5 % in group A and 81.2 % in group B; and compliance was better in group B. Digestive system adverse effects were less frequent in group B than in group A. Median follow-up time was 2.8 years; 3-year survival rate was 69.6 % in group A and 87.3 % in group B; and 3-year RFS rate was 76.4 % in group A and 73.1 % in group B. CONCLUSIONS Our data show improved compliance and fewer adverse effects with alternate-day S-1 administration, which appears to be a more sustainable option for adjuvant chemotherapy for Stage II or III gastric cancer.