Aneurysmal dilatation of a patent ductus arteriosus is a rare condition. One hundred and fifty three cases have been reported, of which 59 (39%) were operated, with an operative mortality of 16%. The authors report a case detected in a premature neonate on chest X-ray showing a mass in the mid-mediastinal region. Surgical resection was undertaken by video-thoracoscopy. Peroperative analysis of the resected specimen suggested a vascular structure and this was confirmed by histological study, which showed spontaneous aneurysm of a thrombosed patent ductus arteriosus or cystic residue of a patent ductus arteriousus. The authors underline the importance of peroperative puncture of structures of uncertain origin and the possibility of conversion of such a procedure to that of an extreme emergency.