A 46-year-old man presented with shortness of breath and recurrent, left-sided pleural effusions. A computed tomography (CT) scan of the chest showed a large, left-sided pleural effusion with a mass in the posterior mediastinum. A pleural tap showed lymphocytic exudate. The biopsy of the mass was inconclusive. A left posterolateral thoracotomy was performed, which showed a large posterior mediastinal mass adherent to the left lung. The mass was completely excised and the histopathology proved it to be giant predominantly cystic schwannoma. The pleural effusion resolved after tumor resection.