The unique case of a child with idiopathic fibrosing mediastinitis mimicking neoplasm is presented. A 5-year-old boy presented with pneumonia and was found to have a complex, heterogeneous, and calcified mediastinal mass along the left hilum. Percutaneous and surgical biopsies, while suggesting a potential epithelial malignancy, were nonconclusive. Owing to worsening symptoms of airway obstruction and chest wall invasion, resection was performed for therapeutic and diagnostic purposes. This ultimately required pneumonectomy on cardiopulmonary bypass. Pathology revealed fibrosing mediastinitis with infiltration of lung parenchyma, and subsequent workup for infectious, neoplastic, granulomatous, and autoimmune etiologies was negative.