ABSTRACT
A teratoma is the most common benign germ cell tumor that develops in the mediastinum. Patients with a mediastinal teratoma are usually asymptomatic. However, a spontaneous rupture of a mediastinal teratoma into the pleural cavity or adjacent organs can cause severe chest pain, hemoptysis, acute dyspnea, etc. Complications such as recurrent pneumonia, pericardial effusion, pleural effusion and great vessel invasion can sometimes occur. We encountered a case of a patient with an abrupt onset of dyspnea after persistent shoulder pain for one month. The X-ray examinations revealed a unilateral mediastinal mass with contralateral pleural effusion. Subsequent evaluations confirmed a spontaneous rupture of the teratoma into the contralateral pleural cavity.