We report here a boy with epilepsy and congenital heart defect, complicated postoperatively by complete atrioventricular (A-V) block caused by an adverse effect of carbamazepine (CBZ). He had been taking CBZ for 7 years to treat complex partial seizures. He also had endocardial cushion defect and first-degree A-V block, and underwent cardiac surgery at the age of 17 years. The postoperative course was unremarkable except transient complete left bundle branch block occuring one day after the surgery. Oral CBZ (400 mg per day) was continued. Five days after the surgery, bradycardia (20 beats per minute) suddenly developed, and electrocardiography (ECG) showed complete A-V block. Pervenous pacing was begun, and the heart rate gradually recovered. CBZ was discontinued on the suspicion that it caused the arrhythmia, although its serum level was estimated to be within the therapeutic range (4 to 5 microg/ml). He underwent pervenous pacing for 12 days. He was discharged 27 days after the surgery, when ECG returned to first-degree A-V block. In this case, the cardiac conduction system was affected by an adverse effect of CBZ, in combination with the preoperative first-degree A-V block and the effects of cardiac surgery, resulting in complete A-V block. Although reports of similar cases are scarce, caution should be made in prescribing CBZ to patients who either have cardiac conduction abnormalities or undergo cardiac surgery.