A 65-year-old woman was referred to our department for investigation and treatment of hoarseness. A chest computed tomography (CT) scan showed a mediastinal mass spreading into the aortopulmonary window. This finding and that of flexible laryngoscopy suggested that the hoarseness was being caused by left recurrent nerve involvement resulting in left vocal cord paralysis. Thus, we performed a mediastinoscopic biopsy via a parasternal incision. Pathological examination revealed dense fibrous tissue infiltrated with varied inflammatory cells. Because no etiological pathogen or neoplastic lesion was identified, we diagnosed idiopathic fibrosing mediastinitis and began treating the patient with prednisolone. After a course of treatment, the mediastinal lesion showed a remarkable response. The hoarseness resolved as the lesion became smaller. Laryngoscopy confirmed recuperation of vocal cord function. This report shows that steroid therapy is a treatment option for hoarseness caused by recurrent laryngeal nerve involvement of fibrosing mediastinitis, if administered under close observation.