We performed intraoperative peritoneal cytology in 171 gastric cancer patients undergoing curative surgery. Intraperitoneal free cancer cells were demonstrated in almost all patients in whom the area of serosal cancer invasion exceeded 15-20 cm2. In patients with both serosal cancer invasion and free cancer cells the 5-year survival rat was 13% as compared with 85% for patients who had neither, and 40% for patients who had serosal invasion but no free peritoneal cancer cells. Peritoneal metastasis was the most frequently observed recurrence pattern. Therefore, in gastric cancer patients with marked serosal invasion, intraoperative IP administration of cytocidal anticancer drugs should be considered.