Cystic lesions located in the gastric wall are a rare finding in endosonography of the gastrointestinal tract. Compared to all forms of benign and malignant tumours of the stomach, gastric duplication cysts are an uncommon anomaly--especially in adults. We report on a 59-year-old woman suffering from intermittent abdominal pain, weight loss and nausea. A gastric duplication cyst was identified by endoscopic ultrasound (EUS), but malignancy was excluded by EUS and fine-needle aspiration histology. Because of continuously increasing abdominal complaints, surgery was performed with partial resection of the gastric corpus and splenectomy. Gross anatomy and histology showed a gastric cyst measuring 150 mm in maximum diameter with no evidence of malignancy or inflammation. Following surgery, the patient's condition recovered fully.