We report a case of a 45-year old woman who was found to have a giant mediastinal tumour with radiological degenerative changes. She underwent thoracotomy to remove the mass, which was eventually diagnosed histologically as an ancient schwannoma, whereas cytological interpretation of the accompanying pleural fluid was malignant. Ancient schwannoma is a rare variant of schwannoma, histologically showing atypical features that may result in erroneous diagnosis of a malignant tumour. Clinical and radiological findings are important aids for further consideration of surgical removal of these potentially resectable tumours.