Cowden's disease, a rare autosomal dominant disorder characterized by benign hamartomatous overgrowth of various tissues, increases the risk of cancer of the thyroid, breast, endometrium, prostate, and possibly other organs. Generally, germline mutations in the coding sequence for PTEN are found in 80% of patients with Cowden's disease. Here we report a rare case of incidentally discovered gastric polyposis during esophagogastroscopy for medical screening in a patient with a history of surgery for breast and thyroid cancer. Identifyng the mutation in the PTEN gene to a diagnosis of Cowden's disease. (Korean J Gastrointest Endosc 2010;40:361-365)