15002 Background: Gastric cancer patients with free cancer cells in the peritoneal cavity as detected by cytologic examination of the peritoneal washes (CY1) are known to have dismal prognosis. In vivo studies by the authors suggest prognostic impact of early postoperative treatment with S-1 (Nakanishi H, et al. Cancer Sci 2003;94:112–8) . No standard treatment for this population has been proposed to date. Methods: A phase II study was conducted to evaluate the efficacy of S-1, a new oral fluoropyrimidine, given postoperatively to patients with CY1 disease. Eligibility criteria included histologically confirmed adenocarcinoma of the stomach without distant metastasis (peritoneal seeding allowed if completely resected), curatively resected with negative margins, <80 years of age with adequate organ functions and preoperative PS of 0∼2, and written informed consent. The primary endpoint was 2-year survival rate and secondary endpoints were time to treatment failure and safety. By referring to the historica...