We investigated the utility of intraoperative peritoneal lavage smears and gastric wall brushing smears for diagnosis of peritoneal dissemination and serosal involvement during gastric cancer surgery. One hundred fifty-three patients with gastric cancer were examined; 138 who underwent gastric resection and 15 who did not. The incidence of cancer cell positivity determined by these methods was 8.0% in resected cases, and 80.0% in unresected cases. According to macroscopic serosal invasion, the incidence of positive cytology was 0% for S0, 15.4% for S1, 13.0% for S2, and 38.7% for S3. According to microscopic wall invasion, the incidence of positivity was 0% for m and sm, 6.3% for mp, 0% for ss, and 22.2% for se and si. In terms of macroscopic peritoneal dissemination, the incidence of positivity was 2.6% for P0, 28.6% for P1, 42.9% for P2, and 90.0% for P3. These results suggest that peritoneal lavage smears and gastric wall brushing smears are useful for predicting the spread of peritoneal dissemination. Thus, further study of intraoperative fast-smear cytology seems warranted.