Cytologic examination in the lavaged peritoneal fluid during surgery was performed in 125 patients with gastric cancer. Immediately after laparotomy, 50ml of saline solution was poured into the Douglas pouch and the left subphrenic cavity. Precipitation of collected fluid was stained by Papanicolaou's procedure, and according to Papanicolou's classification class N and V were judged as positive cytology. The incidence of the positive cytology was 28.8% in all patients, and 50.8% in stage N patients. It was closely correlated with such factors as peritoneal dissemination (P), histological depth of wall invasion, serosal invasion (S), area and gross findings of serosal invasion. The number of cases with microscopic intraperitoneal free cancer cells without peritoneal dissemination were 10 (11.8%), and their survival curves were lower than those without free cancer cells nor peritoneal dissemination. The incidence of the positive cytology was higher (60.5%) in patients whose areas of serosal invasion were over 10cm2 and macroscopically granular, tendon like or mossy like, and in patients whose areas of serosal invasion were over 25cm2 and scar like. It was suggested that peritoneal lavage cytology was useful to reveal the potential cases of disseminated peritoneal metastaiss and might be helpful for prophylaxis against peritoneal dissemination and forcast of recurrece.