AbstractMesenteric cysts are uncommon intra-abdominal lesions that account for only one in every 100 000 adult hospitalizations. Their diagnosis is based on a comprehensive clinical examination as well as radiological modalities such as ultrasonography and computed tomography (CT) scans, and it is usually a clinical challenge because of non-specific symptoms. We present our first case of a 51-year-old man with a simple mesenteric cyst accompanying acute appendicitis diagnosed by CT scan of the abdomen and treated by exploratory laparotomy, complete enucleation of the cyst and appendectomy with a 10 month follow-up without complications or recurrence. This type of presentation has not been thoroughly investigated, with only two children reported during our literature review. Even if there is a high level of suspicion, a CT scan is required for confirmation.