The benign cystic mesothelioma of the peritoneum is a rare lesion and is known for local recurrence. This is first case report of a rapidly developing massive abdominal tumor with histological finding of benign cystic mesothelioma (BCM). We describe a BCM arising in the retroperitoneal tis[sue on the right side, lifting ascending colon and cecum to the left side of abdomen. Patient was an active 58-year-old man who noticed a rapid abdominal swelling within a two month time period with a weight gain of 40 pounds. Patient had no risk factors including occupational (asbestos, cadmium), family history, social (alcohol, smoking) or history of trauma. We will discuss the clinical, radiologic, intra-operative, immunohistochemical, pathologic findings, and imaging six months after surgery. Patient has no recurrence and no weight gain on follow up visits and imaging.