Mesenteric cysts are rare cystic malformations of the mesentery. They are usually located at the iliac mesentery. Clinically most mesenteric cysts are asymptomatic, but sometimes they present with non-specific abdominal symptoms. Diagnosis can be aided using US, CT and MRI but careful interpretation of the images and high index of suspicion of this rare condition is essential for the correct diagnosis, which cannot always be preoperatively established. The therapeutic method of choice is complete surgical excision of the cyst which minimizes the possibility of recurrence. Histopathologically they are classified in six group. We present a case of a mesothelial mesenteric cyst in patient with colon cancer. The cyst was misdiagnosed as urinary bladder diverticulum in the preoperative CT scan.