Fibrosing mediastinitis is rare. One type of this disease is idiopathic fibrosing mediastinitis. It is necessary to rule out malignancy in order to accurately diagnose fibrosing mediastinitis. We herein report a case of anaplastic large cell lymphoma diagnosed three months after a preliminary diagnosis of fibrosing mediastinitis. Glucocorticoid therapy was not successful in controlling disease progression. Immediately after initiating chemotherapy for lymphoma, the patient's symptoms improved dramatically and the mediastinal lesion decreased in size. Although few similar cases have been reported, hidden malignancy may present as fibrosing mediastinitis. Therefore, physicians should consider the probability of malignancy in patients with fibrosing mediastinitis because treatments may vary accordingly.