Most mediastinal teratomas are histologically well-differentiated tumors and benign. The majority of patients with a mediastinal teratoma are asymptomatic and their tumors are usually discovered incidentally on chest radiography. On rare occasions this tumor will rupture spontaneously into the adjacent organs. A 72-year-old female patient was admitted for dyspnea and she had a multiloculated pleural effusion in the left lung field. Although repeated pleural biopsy and pleural fluid cytology did not prove the presence of malignancy, we assumed that this was a malignant effusion because it revealed consistently high levels of carcinoembryonic antigen and carbohydrate antigen 19-9, and the chest CT scan did not show typical fat or bone density in the mass. Secondary infection and an uncontrolled septic condition due to pleural empyema finally compelled the patient to undergo a surgical operation. Mature teratoma was the final diagnosis and she has done well without recurrence for 2 months.