Castleman's disease is characterized by the presence of a rare benign tumor of lymphoid origin which usually arises in the mediastinum but rarely occurs in the retroperitoneum. In the radiology literature, the unenhanced CT appearance of the disease has been decribed as hypodense or isodense as compared to that of the liver, and as showing homogeneous contrast enhancement. In this case, CT revealed a well-circumscribed, homogeneous enhancing mass with an internal cystic portion, a finding different from those described in the literature. We report a case of retroperitoneal Castleman's disease in which there was unusual mucoid degeneration resulting in a large cystic component, and which has not been previously reported.