Because the automatic internal cardioverter defibrillator's long-term ability to reduce arrhythmic mortality in patients with ventricular tachycardia/fibrillation is unknown, it is important to determine whether the threshold for defibrillation changes over time. Serial defibrillation thresholds were measured in 23 patients over a mean replacement time of 24.8 +/- 7.5 months. In all cases the lead system was a superior vena cava coil to a left ventricular epicardial patch. The defibrillation threshold for the entire group increased from 12.3 +/- 4.7 J to 16.9 +/- 5.9 J (p less than 0.05). Striking increases in the defibrillation threshold were seen in the subgroup of patients taking amiodarone (from 10.9 +/- 4.3 J at implantation to 20.0 +/- 4.7 J at replacement, p less than 0.05). Defibrillation threshold decreased in patients taking no antiarrhythmic drugs or taking class I agents. Thus, the increase in mean defibrillation threshold was the result of an increase in the patients taking amiodarone. These data suggest that at initial implantation lead systems associated with the lowest defibrillation threshold should be used and the defibrillation threshold should be measured at generator change to guarantee an adequate margin of safety.