We present clinical, haemodynamic, and angiographic data on a 48-year-old woman with a fistulous aneurysm of the ductus arteriosus. This is the first reported case of a spontaneous fistulous aneurysm of the ductus arteriosus in an adult in which an antemortem diagnosis was made. This lesion typically presents with shortness of breath, hoarseness caused by recurrent laryngeal nerve paralysis, and chest pain, and poses a critical threat to life because of progressive compression, erosion, and haemorrhage into the oesophagus or tracheobronchial tree. Fistulous aneurysm of the ductus arteriosus should be considered in the differential diagnosis of an enlarging mediastinal mass, especially when a continuous heart murmur is noted. Early definitive diagnosis and surgical repair in adults are essential for survival.