A cytological examination in the lavaged peritoneal fluid during operation was performed in 462 patients with gastric cancer. The incidence of positive cytology was 36% in serosal invasion cases. The prognosis of positive cytology cases without peritoneal dissemination was slightly better than the P (+) cases. The incidence of the positive cytology was 48% in patients with serosal invasion more than 16 cm2. The prognosis of patients with serosal invasion below 32 cm2 was significantly better than in the patients with above 32 cm2. Intraperitoneal repeated administration of MMC 10 mg/body and CDDP 40-50 mg/body was continued one a month via the drug delivery system. Negative change of cytology was observed in 7 out of 14 patients, and the prognosis of these patients was better than that of the unchanged cytology group. Side effects such as severe gastrointestinal symptoms and bone marrow toxicities were not observed. Further studies should be done to confirm the efficacy of the intraperitoneal chemotherapy in terms of the prolongation of life.