The standard implantable defibrillator waveform is a truncated exponential of approximately 6 msec duration. This study compares the defibrillation efficacy of a standard monophasic truncated exponential to a biphasic 12 msec truncated exponential waveform in 21 patients undergoing automatic implantable cardioverter defibrillator (AICD) surgery. For the biphasic waveform, the polarity was reversed and remaining capacitor voltage was attenuated by 75% after 6 msec. Two hundred thirty episodes of VF were induced with 115 "matched pairs" of monophasic and biphasic waveforms of identical initial capacitor voltages given over a range from 70 to 600 V (0.35 to 25.7 joules). The biphasic waveform was superior to the monophasic waveform (p less than 0.006), especially for "low energy" defibrillation. For initial voltages less than 200 V, the percent successful defibrillation was 28% for the monophasic waveform versus 64% for the biphasic waveform and from 200 to 290 V (energies less than 6.4 joules) it was 45% versus 69%. There was no difference in the two waveforms in time to the first QRS complex or in the blood pressure following defibrillation. This study shows that a 12 msec biphasic truncated exponential is superior to a 6 msec monophasic waveform for defibrillation in man, especially at energies less than 6.4 joules. The waveform can be achieved in an implanted device without any increase in capacitor size or in battery energy consumption.