The area of serosal cancerous invasion and intraperitoneal free cancer cells were examined in 360 gastric cancer cases. The positivity rate of intraperitoneal free cancer cells was 20% in the group with under 20 cm2 of serosal invasion, 59% with 20 cm2 to 30 cm2, and 76% over 30 cm. The postoperative survival rate was significantly lower in the group with free cancer cells than in the group without them. Prognosis was better in case with the area of serosal invasion was smaller. Hematogenous metastasis was frequent under 20 cm2, but over 20 cm2, peritoneal metastasis predominated.