A pretherapeutic staging system to design operative or neoadjuvant treatments in gastric cancer is needed. In this study, a simple staging system based on diagnostic laparoscopic findings under local anesthesia to define a treatment was developed. This study was conducted with 68 patients with advanced gastric cancer. All the patients had been diagnosed as more than T3, or were suspected to have peritoneal seeding. Preoperative diagnostic laparoscopy under local anesthesia was performed in the operation room. After insertion of the trocars, the abdominal cavity was inspected, and biopsy and/or abdominal lavage sampling was performed. Patients were allocated into three stages based on laparoscopic and histopathologic findings. Twenty-eight out of 68 patients were diagnosed as P1, 34 patients as P0CY0 and 6 as P0CY1. Based on these results, 57 patients underwent operation. The accuracy rate of diagnosis was 95% in the P category, and 93% in the CY category. Lavage telomerase activity was examined on 10 patients with P0CY0 gastric cancer. Two out of 10 were positive for this activity, and both of them had died due to peritoneal recurrence within a year after operation. In conclusion, the proposed diagnostic laparoscopic staging system is a simple and reproducible method for selection of a suitable therapy. It is considered that diagnostic laparoscopy under local anesthesia is a useful preoperative examination in cases of advanced gastric cancer. Moreover lavage telomerase activity may be a more sensitive predictor of peritoneal recurrence than lavage cytology.