Paclitaxel was used as the first-line drug for treatment of a case with peritoneal recurrence of gastric cancer, accompanied by cancerous ascites. Because paclitaxel was ineffective, TS-1 was used as the second-line drug, resulting in disappearance of cancerous ascites on diagnostic imaging. This case was an 81-year-old male patient. In February 2004, he underwent resection of the pyloric side of the stomach (D 2) based on a diagnosis of advanced gastric cancer. CT scans, conducted in February 2005, revealed ascites, and a diagnosis of cancerous peritonitis was made on the basis of subsequent cyto-diagnostic findings. He was later hospitalized because of anorexia and difficulty with oral ingestion, and received paclitaxel therapy (60 mg/m(2)). Abdominal CT scans in May of the same year showed the disappearance of ascites. Thereafter, he was managed as an outpatient. In June of the same year, relapse of ascites was detected by CT scans, and exacerbation of ascites was seen in August. In October, paclitaxel was switched to TS-1 (80 mg/m(2)). CT scans, obtained at the end of two cycles of TS-1 therapy, revealed complete disappearance of ascites. This therapy was administered for 4 cycles in total. CT scans, performed at the end of each cycle of TS-1 therapy, confirmed the absence of ascites. At present (July 2006), the patient is managed on an outpatient basis. Our experience with this case suggests that if paclitaxel therapy fails to exert satisfactory efficacy, switching to second-line TS-1 therapy is a promising means of treating gastric cancer complicated by cancerous peritonitis.