Background - It has been reported that triphasic defibrillation waveforms cause less myocardial injury than biphasic waveforms. This study compared the defibrillation thresholds (DFTs) of triphasic and biphasic waveforms. Methods and Results - DFTs were determined for a transvenous lead system and a 300-μF-capacitor defibrillator. In 8 pigs (group 1), DFTs were determined for 5 triphasic waveforms with tilts of 80%, 83%, and 86% and for 1 biphasic waveform. DFTs were determined in another 8 pigs (group 2) for 2 triphasic and 4 biphasic waveforms with tilts of 43%, 49%, and 56%. In both groups, a biphasic waveform from a 140-μF-capacitor defibrillator was also evaluated, and both shock polarities were tested for each waveform. In group 1, with the 300-μF-capacitor defibrillator, the leading-edge voltage and energy stored at DFT were significantly lower for triphasic waveforms with phase-duration ratios of 50/33/17 and an anode at the right ventricular electrode for phase 1 than for biphasic waveforms (P < 0.001). [n group 2, the stored energy of triphasic waveforms with 56% and 49% tilt was significantly lower than that of biphasic waveforms with the same tilts for anodal but not cathodal phase 1 at the right ventricular electrode. Electrode polarity significantly affected the DFT of triphasic waveforms for both studies. Conclusions - Some 80% tilt triphasic waveforms defibrillate more efficiently than biphasic waveforms with a 300-μF-capacitor defibrillator. The triphasic waveforms for both groups were not superior to 140-μF- capacitor biphasic waveforms. The efficacy of triphasic waveforms depends on phase durations and electrode polarity.