The intraoperative diagnosis of a persistent left superior vena cava (LSVC) that drains into the left atrium (LA) via the left superior pulmonary vein may be difficult because of its extrapericardial pathway. We report here the case of a 48-year-old man, operated for a mitro-aortic endocarditis complicating a Laubry-Pezzi syndrome. The opening of the LA was followed immediately by a sudden air lock. It was only after opening of the left pleura that we were able to find and then ligate the LSVC. The patient underwent mitral valve repair, closure of the infundibular septal defect and aortic valve replacement.