Patients with mediastinal teratoma are usually asymptomatic, but may develop symptoms by rupture into adjacent structures which result in pneumonia, hemoptysis, pleural effusion, pericardial effusion, or pneumothorax. Rarely, life-threatening acute respiratory distress require a emergenc y surgery. Rupture into pleural cavity may result in pleuritis and pleural effusion with severe anterior chest or back pain. The symptom must be differentiated from other common intrathoracic distress diseases. Clinical, cytologic and radiologic examinatio ns of pleural effusion, and moreover, measurement of enzymes such as amylase or insulin, which is secreted from pancreatic tissues, in pleural effusion and cystic fluid enabled us to make the diagnosis of rupture of mediastinal teratoma preoperatively.