Objectives: The purpose of this study was to determine if there is a difference in commercially available biphasic waveforms. Background: Although the superiority of biphasic over monophasic waveforms for external cardioversion of atrial fibrillation (AF) is established, the relative efficacy of available biphasic waveforms is less clear. Methods: We compared the effectiveness of a biphasic truncated exponential (BTE) waveform and a biphasic rectilinear (BR) waveform for external cardioversion of AF. Patients (N = 188) with AF were randomized to receive transthoracic BR shocks (50, 75, 100, 120, 150, 200 J) or BTE shocks (50, 70, 100, 125, 150, 200, 300, 360 J). Shock strength was escalated until success or maximum energy dose was achieved. If maximum shock strength failed, patients received the maximum shock of the opposite waveform. Analysis included 141 patients (71 BR, 70 BTE; mean age 66.5 ± 13.7. Forty-seven randomized patients were excluded because of flutter on precardioversion ECG upon blinded review (n = 25), presence of intracardiac thrombus (n = 7), or protocol deviation (n = 15). Groups were similar with regard to clinical and echocardiographic characteristics. Results: The success rate was similar for the two wave forms (93% BR vs 97 BTE, P = .44), although cumulative selected and delivered energy was less in the BTE group. Only AF duration was significantly different between successful and unsuccessful patients. No significant complications occurred. Conclusions: Biphasic waveforms were very effective in transthoracic cardioversion of AF, and complication rates were low. No significant difference in efficacy was observed between BR and BTE waveforms. Impedance was not an important determinant of success for either biphasic waveform.