Background Peritoneal dissemination is frequently found during laparotomy in patients with serosa-invading gastric cancer. Detection of exfoliated cancer cells in abdominal lavage cytology is indicative of stage IV because of its strong association with peritoneal dissemination. Herein we have described peritoneal lavage cytology using a bedside procedure under local anesthesia. Methods A prospective study of 113 patients with serosa-invading gastric cancer but without peritoneal metastases was performed. A drainage tube was inserted into the abdominal cavity for peritoneal lavage. Patients with negative cytology (CY0) were scheduled for curative gastrectomy. Results The bedside procedure was performed safely without any complications. Lavage cytology identified CY1 in 35 (31.0%) patients and CY0 in 78 (69.0%) patients. Patients with CY0 underwent laparotomy and peritoneal lavage cytology, and 9 were found to have peritoneal disease (3 with operative CY1, 4 with peritoneal dissemination, and 2 with both operative CY1 and peritoneal dissemination). Two other patients had small, distant metastases. Finally, curative gastrectomy was achieved in 67 (59.3%) patients, but not in 46 (40.7%) patients. Thus, our bedside, pre-operative peritoneal lavage detected 76.1% (35/46) of noncurative disease before operative with a false-negative rate for detecting peritoneal disease of 20.5% (9/44). Patients with pre-operative CY1 had a poorer prognosis than pre-operative CY0 (2-year cause-specific survival 26.6% vs 82.6%). Conclusion Pre-operative bedside peritoneal lavage under local anesthesia followed by cytology is a simple and safe method for the pre-operative diagnosis of peritoneal dissemination and may help to reduce unexpected, noncurative surgery.