The optimal interval for drug administration was examined for repeated intraperitoneal chemotherapy (IPC). The subjects were 31 patients who underwent curative surgical resection excluding P1 or CY1 factor, followed by IPC. IPC was carried out 180 times in total, and the intervals were divided into three groups: 2-week interval 45 times, 3-week interval 10 times and 4-week or greater interval 94 times. The optimal method of drug administration was evaluated from the therapeutic outcome and the development of adverse effects with each interval time. The cytology of ascites obtained via an i.p. port was examined before each drug administration, and those with a negative change (CY0) were judged as responders. The adverse effects in the patients receiving drugs at 2-week intervals were grade 1 or 2, although the incidence was high compared with other patients. All responders obtained a negative change in CY within six courses. At present, we conclude that it is reasonable for IPC to be carried out six times at 2-week intervals.