The association of intraventricular conduction defects and aortic valvular disease is widely recognized. This study was undertaken to evaluate the effects on survival of left bundle conduction defects (LBCDs) as a consequence of aortic valve replacement. A total of 133 patients were followed between 1 and 70 months after operation, with a mean follow-up of 32.1 months. The incidence of intraoperative LBCDs was 31.6% or 42 patients. There were 13 deaths in the group of 42 patients with LBCDs compared to eight deaths in the group of 91 patients without such abnormalities (p less than 0.01). Sudden death occurred in five of 42 patients with postoperative LBCDs and two of 91 patients with normal intraventricular conduction (p less than 0.025). The survival rate in these patients with significant aortic stenosis and normal intraventricular conduction was 89.9%, whereas if LBCD had occurred after operation, the survival rate was 65.7% (p less than 0.005). If the LBCD was accompanied by a left axis deviation, the survival rate was 21.7%. Sudden death may be due either to a tachyarrhythmia or perhaps to progression from LBCD to complete heart block or trifascicular block. It is important that this group of patients be monitored closely after operation. There may be an indication to insert prophylactic permanent pacemakers in this group.