We experienced a 5-month-old infant with mediastinal bronchogenic cyst manifested with severe dyspnea. Chest x-ray revealed no abnormal shadow in the mediastinum, but this disease was suspected from a deviation of the esophagus. The definitive diagnosis was established by computerized tomography (CT) and echography. The infant's life was saved by emergency surgery. Although the diagnosis of this disease is difficult, late diagnosis may lead to an unfavorable prognosis. CT and echography are very useful for diagnosis of this disease.