We report a patient with peritoneal metastases who was successfully treated with a novel oral fluoropyrimidine anticancer drug, TS-1, as first-line chemotherapy. The patient was a 72-year-old man who had undergone curative resection for type 4 gastric cancer with peritoneal dissemination that was located only at the greater omentum. Final findings were P1, T4, N1, and stage IV. Eighteen months after the gastrectomy, his cancer recurred with peritoneal metastases; these were diagnosed by the presence of multiple stenoses of the colon, an abdominal mass, and elevated levels of the serum tumor markers, carcinoembryonic antigen (CEA) and carbohydrate antigen (CA)19-9. He was treated with 100 mg/day of TS-1, given orally, for 28 days, followed by 14 days' rest, as one course. Two courses of the treatment resulted in a marked reduction of the abdominal tumor, without severe toxicity. After 3 courses, barium enema revealed dramatic improvement in the stenoses. Laparoscopy after 11 courses showed neither peritoneal dissemination, nor ascites, and peritoneal lavage cytology was negative. The serum tumor markers were reduced to almost normal levels. Two years after the onset of the peritoneal metastases, the patient is alive without any sign of progressive disease. Our report is the first to demonstrate the advantages of TS-1 as chemotherapy for the treatment of peritoneal metastasis of gastric cancer.