Two fairly similar cases of internal mammary arterio-venous fistula were observed in two patients involved in serious road accidents, following which a catheter was inserted into the subclavian vein for the purposes of resuscitation. The clinical diagnosis was made in both cases on routine examination, which revealed a continuous right subclavian bruit with extensive radiation three years after the accident. There were no functional implications. Further investigations revealed a fistula located between the internal mammary artery and the right brachiocephalic venous trunk and demonstrated the haemodynamic consequences: normal intra-cardiac pressures, but a pulmonary blood flow which was twice that of the systemic blood flow. Both cases were treated by means of surgical cure via thoracotomy, which confirmed the nature of the arterio-venous fistula and which defined the relations with the phrenic nerve, which is the most exposed element. The results of this operation were excellent in both cases.