A 40-year-old woman was admitted to the hospital with general fatigue and cough. Chest CT films revealed mediastinal lymphadenopathy with multiple low density areas, but no pulmonary lesions. There were no abnormal findings on neck, abdominal or pelvic CT. A PPD skin test was strongly positive, but M. tuberculosis bacilli were not found in the sputum. Mediastinal tuberculous lymphadenitis was diagnosed histologically and bacteriologically from specimens obtained by mediastinoscopy. Fiberoptic bronchoscopy did not reveal tracheobronchial tuberculosis. Follow-up chest CT three months after the start of antituberculosis chemotherapy with isoniazid, rifampicin and ethambutol hydrochloride showed that the mediastinal lymph nodes were decreased in size. Mediastinal tuberculous lymphadenitis in adults is rare, but the number of reports has increased. Mediastinal tuberculous lymphadenitis in adults must be distinguished from other causes of mediastinal masses. In this case, mediastinoscopy was very useful for differential diagnosis.