Introduction: S-1 has been shown to be an effective adjuvant treatment option for East Asian patients who underwent gastrectomy for stage II/III gastric cancer. We conducted a phase I/II study to evaluate the feasibility, tolerability, and efficacy of administering S-1 in the adjuvant setting after R0- resection of adenocarcinoma of the stomach and esophagogastric junction (EGJ) in Caucasian patients. Methods: In this single-cohort, open-label, phase I/II trial, we enrolled patients with locally advanced adenocarcinoma of the stomach or EGJ having undergone R0-resection with or without neoadjuvant treatment. One treatment cycle consisted of oral S-1 (30 mg/m2 bid) for 14 days. Cycles were repeated every 3 weeks for 18 cycles (54 weeks). Primary endpoint was feasibility and tolerability. Safety was evaluated according to the Common Toxicity Criteria Adverse Events (CTCAE) version 4.0. Secondary endpoints were 1-year relapse-free survival (RFS) rate, RFS, and overall survival (OS). Results: Between October 2015 and February 2018, 32 patients were enrolled in 12 German centers, and 30 started adjuvant study treatment. Seventeen patients completed all 18 cycles. Two patients terminated study treatment early due to adverse events (AEs), 7 due to patient?s or investigator?s decision, and 4 due to recurrence or distantmetastasis during adjuvant therapy. Dose levels were reduced to 25 mg/m2 in 9 patients and to 20 mg/ m2 in 1 patient. Of patients completing all 18 cycles, 5 did so with reduced dosage of S-1. Documented grade 3 AEs were neutropenia, diarrhea, vomiting, polyneuropathy, palmarplantar erythrodysaesthesia, and rash. Serious AEs were observed in 7 patients. Median RFS was 32.2 months. One-year RFS ratewas 77%. Data on OS were still premature at the end of the study. Conclusion: Adjuvant treatment with S-1 for 1 year is a feasible and safe treatment option for Caucasian patients diagnosed with gastric adenocarcinoma or cancer of the EGJ after R0-resection. ? 2024 The Author(s). Published by S. Karger AG, Basel.