A 54-year-old male, non smoker, began to suffer from persistent dyspnoea and fever. X-ray and CT scan showed a cystic lesion located in the left anterior mediastinum. This lesion was removed instead of the initially scheduled surgical resection with video-assisted thoracoscopic surgery (VATS) preceded by fine needle aspiration (FNA). A diagnosis of thymoma arising in the wall of thymic cyst was made. Such a rare tumour should be taken into consideration in treating patients with a cystic mediastinal lesion, before VATS and FNA.