A case of gastric duplication associated with marked atypism in an adult is reported. The patient was a 62-year-old woman presenting a mass in left upper quadrant of abdomen. The mass was a closed gastric cyst located in the greater curvature, measuring 11 9 cm in diameter. The cyst had common muscular layer with stomach. It showed gray-white firm cystic wall with yellowish brown soft necrotic tissue. Microscopically, the cyst was lined by columnar mucin-secreting epithelium having marked cellular atypism. The cyst wall was supported by layers of smooth muscle, accompanied with severe fibrosis and chronic inflammation.