Among 142 cases of bacterial endocarditis (BE), complete heart block (CHB) was found in six cases (4%) and first-degree (1°) or second-degree (2°) A-V block in 14 cases (10%).The aortic valve was involved in 18 of 20 cases with atrioventricular (A-V) conduction disturbance, including all six cases of CHB.Anatomic observations (four autopsy, one operative) were made in five of the six cases of CHB. In these cases, a common finding, in addition to involvement of the aortic valve, was extension of the infection to adjacent structures resulting in cardioaortic fistulae. CHB likely resulted from extension of infection to the major conduction tissues.Five of the six patients with CHB died suddenly while in the hospital. One patient was treated with electric pacing while the infection was being controlled and, 38 days later, underwent successful replacement of the aortic valve. Conduction abnormalities are important possible complications of aortic valvular BE. Prompt pacing may be a lifesaving procedure, allowing eradication of infection as a prelude to surgical therapy.