A 63-year-old woman who had been treated for myelodysplastic syndrome had a high fever. Chest radiography and computed tomography revealed a giant tumorous shadow from the right mediastinum to the hilum. She was treated with antibiotics but with little effect. The culture of bronchial washing fluid and sputum revealed Mycobacterium tuberculosis infection. There were no malignant cells in the sputum or bronchial washing fluid. Tumor markers were within normal limits. She was treated with isoniazid, rifampicin, streptomycin and pyrazinamide. On the sixth day of treatment, high fever disappeared, and sputum culture for tuberculosis became negative after two months. The size of the mass decreased with clinical improvement. The final diagnosis was pulmonary tuberculosis with mediastinal lymphadenopathy. Since the patient was a compromised host, the giant mass was considered to be induced by atypical response to mycobacterial infection.