Alimentary tract duplications are uncommon congenital anomalies that are usually present during the first decade of life. However, a smaller number of cases may remain unsuspected until adulthood. They are most common in the ileum but can occur anywhere along the alimentary tract. Duplications may be cystic or tubular in appearance and characteristically arise from the mesenteric aspect of the intestine. Abdominal pain, nausea and/or vomiting, palpable mass, weight loss, and bleeding are the most common symptoms. This paper reports the case of a 24-year-old male with a cystic duplication of the ascending colon not correctly diagnosed prior to operation. When faced with such an abdominal cystic lesion, differential diagnoses versus duplication cyst, mesenteric cyst, choledochal cyst, giant diverticulum, and cystic tumour of the pancreas should be considered. Ultrasound and computerized tomography are good tools for diagnosis and for delineating the relationship between the cyst and peripheral structures. Surgery provides treatment and allows a final pathological diagnosis. Removal of the cyst could be considered satisfactory but resection of the duplication and the adjacent bowel is recommended because of the possibility of malignant degeneration and the risk of gastrointestinal ulceration and haemorrhage due to ectopic gastric mucosa.