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OBJECTIVES To study diagnosis, differential diagnosis, and origin of easily misdiagnosed adult gastric duplication cysts. METHODS Six cases with GDCs were studied using immunohistochemical methods to research the expression of tumor markers. RESULTS Most GDCs were located in the abdominal cavity outside the digestive organs. The cyst wall tissues included a repetition of the full-thickness structure of the stomach wall that was lined by a variety of different epithelia. The expression of CK7 was positive in all epithelia (6/6), while CK7 expression was positive in cardia glands in 5/5 and positive in fundus glands in 1/5. CK 20 was 100% (4/4) positively expressed in the SCE, 100% (3/3) negative in the CCE, and in the SE. It was also expressed positively 100% (5/5) in fundus glands and 80% (4/5) in cardia glands. Both CK7 and CK20 were negatively expressed in the pyloric glands in case 3. The expression of MUC1 was positive in all epithelia and glands, whereas MUC2 expression was negative. MUC5AC was expressed differently in epithelia and glands. CONCLUSIONS GDCs can originate from the antrum or the fundus of the stomach. Tumor markers can help diagnosis and differential diagnosis. This study may help to improve clinical precision treatment.