Purpose: Telomerase activity is generally absent in primary cell cultures and normal tissues. Telomerase is known to be induced upon immortalization or malignant transformation of human cells. Telomerase activity can be increased in immature lymphocytes and activated lymphocytes, but it is not detected in the peripheral blood of normal persons. The authors analyzed peripheral blood telomerase from patients of gastric cancer to evaluate the possibility of using it for diagnosis and as a prognostic factor. Materials and Methods: We obtained blood samples from 11 inflammatory patients and 64 gastric cancer patients. The telomerase activity was measured using the [PCR-ELISA] method. The results were correlated with the T, N, M stage, cell differentiation, vascular, neural, and lymphatic invasion, tumor size, and tumor location. Results: In the 11 inflammatory patients, telomerase activity was not detected while in the gastric cancer patients, a positive rate of $28.1\%$ was noted. The peripheral telomerase activity was not related with tumor size, tumor site, lymphatic and vascular invasion, stage, or histologic differentiation. Conclusion: The peripheral blood telomerase activity for patients of gastric cancer can be utilized as a marker for the diagnosis of not only advanced gastric cancer, but also relatively early stage gastric cancer, but not as a prognostic factor.