We present the MR imaging features in two cases of mediastinal Castleman's disease of the hyaline-vascular type. T1-weighted MR images showed rounded hilar masses of intermediate or high signal intensity with areas of low signal due to vessels and calcification. On T2-weighted images, the masses were of high signal intensity. Mediastinal masses with the above signal characteristics and vascular signal voids may suggest a diagnosis of Castleman's disease.