A 33-year-old female had a left sided chest pain for the last 3 months. Chest X-ray showed a left basal opacity. Computed tomography chest suggested a left sided subpulmonic effusion (17.5x12.2x13 cm) with thick enhanced walls with marked collapse of the left lower lobe and displacement of the heart and mediastinum to the right side. Trial of thoracocentesis was done and it was positive. Trial of intercostal tube insertion was done with a sense of very thick pleura and the patient developed a vasovagal attack. Accordingly, exploratory thoracotomy was decided. Intraoperative assessment showed a huge anterosuperior mediastinal cyst attached to the pericardium and was successfully resected. The pathological findings were compatible with epidermoid cyst.