Neurological accidents following open heart surgery have become exceptionaly rare because of technical advances. However, some minor intellectual problems are quite common (loss of memory, lack of concentration...). The results of previous studies on this subject have been quite contradictory. A prospective study of 40 patients was undertaken to determine the repercussions of open heart surgery with cardiopulmonary bypass on intellectual efficiency. The protocol chosen used Wechsler's intelligence test, the calculation of mental deterioration based on this test and Wechsler's clinical scale of memory. The tests were performed before surgery, in the immediate postoperative period and after six months (30 patients). The purely cardiological side of these cases and social considerations were also taken into account. In the immediate postoperative period, 55% of patients showed signs of intellectual deterioration, with respect to their age. After six months, the overall incidence was 25%. Some patients showed intellectual deterioration before surgery. The influence of the type of surgery is difficult to determine. The cardiopulmonary bypass time was significant. The preoperative intellectual level was also important, as was the return to normal activities after surgery. Finally, age, of course, played an important part. These results cannot be isolated from the study of the psychological reactions of the patients to cardiac surgery and their influence on the outcome and rehabilitation after surgery.