Fibrosing mediastinitis is characterized by abnormal proliferation of acellular collagen and fibrous tissue in the mediastinum. Although most cases in the United States are attributed to Histoplasma capsulatum, there is a different and important idiopathic subset, with potentially different treatment and prognosis implications. We reviewed 12 such cases encountered from 1995 to 2004. Computed tomography showed that the masses were large, averaging 5 x 9 cm, with none showing significant calcification. Five had extension into the neck, and all had some vascular or airway involvement. Mimics may include the precalcific form of postinflammatory mediastinal fibrosis, mediastinal granuloma, malignancy (esp. lymphoma), sarcoidosis, and Castleman disease.