This study, we evaluated the relationship between the macroscopic area of serosal invasion by gastric carcinoma and the incidence of intraperitoneal free cancer cells which were detected by lavage of the Douglas cavity. Between 1976 and 1989, 362 patients with advanced gastric cancer (t3, t4) were gastrectomized and intraperitoneal free cancer cells were recognized in 119 patients (32.9%). Only 17.3% of patients with an area of serosal invasion of less than 10 cm2 had free cancer cells in their peritoneal cavities, however, 68.5% of patients with an area of serosal invasion greater than 20 cm2 had free cancer cells. Even, if curative surgery was performed for patients with free cancer cells, their five-year survival rate was very poor. The five-year survival rate of patients with curatively operated gastric cancer without free cancer cells was 49.3%, however that of patients with free cancer cells was significantly lower (15.4%, P < 0.01). Even when we are not able to receive the cytologic diagnosis for intraperitoneal free cancer cells during the operation, patients with macroscopic areas of serosal invasion greater than 20 cm2 may have free cancer cells in a very high percentage. They should be treated with effective adjuvant chemotherapy the soon after the operation to prevent peritoneal metastatic recurrence and to prolong survival.