Signet ring cell carcinoma is a subtype of adenocarcinoma that is characterized by abundant intracellular mucin accumulation. Diffuse large B-cell lymphoma is the most common histological subtyple of gastrointestinal lymphoma. The gastrointestinal tract is the most frequently involved extranodal site in non-Hodgkin's lymphoma. However, little is known about the coexistence of advanced gastric cancer and gastric diffuse large B cell lymphoma. We report a case of synchronous advanced gastric adenocarcinoma and gastric diffuse large B cell lymphoma in a 62-year-old woman. (Korean J Gastrointest Endosc 2010;40:181-185)