Combined idiopathic retroperitoneal-mediastinal fibrosis is rare. We report a case of mediastinal fibrosis that followed the onset of retroperitoneal fibrosis by six years. A 61-year-old asymptomatic woman was diagnosed with idiopathic mediastinal fibrosis in December of 2006 after discovering a 1.4 cm thick prevascular mass encasing the aortic arch. In August of 2001 the patient had been diagnosed with retroperitoneal fibrosis, which was successfully treated surgically. An axillary thoracotomy found dense adhesions that fixed the arch of the aorta to the adjacent lung. Mediastinal biopsies were consistent with idiopathic fibrosis. We describe the imaging of this case and briefly review the literature.