Mesenteric fibromatosis is an uncommon benign neoplasm occurring in the mesentery or retroperitoneum, and presenting as a firm mass with infiltrative margins and homogeneous parenchyma without necrosis or a cystic component (1-4). Cystic change may occur, usually after prolonged medical treatment, but is extremely rare (5-7). We describe the US and CT findings in a case of mesenteric fibromatosis with spontaneous extensive cystic degeneration.