The effects of high voltage defibrillation shocks given to six swine were studied to determine if there is a limit to the advantage gained from increasing the shock strength. An endocardial electrode was placed in the right ventricle, and a 114‐cm2 cutaneous patch was placed on the left lateral thorax. Monophasic (10 msec) and single capacitor biphasic (5/5 msec) shocks with leading edge voltages of 200, 400, 600, 800. and 990 volts (approximately 2.3–59 J) were tested. For monophasic shocks, the probability of successful defibrillation ranged from 0% at 200 V to 90% at 990 V. The incidence of postshock arrhythmia increased from 0% for successful shocks at 600 V to 67% for successful shocks at 990 V. For biphasic shocks, the probability of success peaked at 97% for the 600‐, 800‐, and 990‐V shocks. The incidence of postshock arrhythmia increased from 8% at 400 V to 55% at 990 V. Although more postshock arrhythmias occurred at lower strengths for bipbasic than for monophasic shocks, an efficacy criterion, quantifying the probability of defibrillation success and the probability that a postshock arrhythmia will not occur, was always higher for biphasic shocks. The probability of success never reached 100% for either waveform while the incidence of postshock arrhythmia increased as the shock strength increased. In conclusion, for the catheter‐patch electrode configuration, increasing the shock strength does not always improve the probability of success and may increase the incidence of postshock arrhythmia. Copyright © 1994, Wiley Blackwell. All rights reserved