A 74-year-old woman was hospitalised for pleuro-pneumonia of the right base. The chest x-ray showed the presence of a right paratracheal opacity which persisted during the course of the infectious episode. A computed tomographic scan of the thorax showed a voluminous anterior mediastinal mass which compressed the trachea without invading it. Surgical excision enabled the ablation of a tumour which was haemorrhagic and the histological examination established a diagnosis of cystic parathyroid adenoma. At anytime did the patient present either clinical symptoms or biochemical signs suggesting primary hyperparathyroidism.