Evidence suggests that biphasic waveforms are more effective than monophasic waveforms for defibrillation in out-of-hospital cardiac arrest (OHCA), yet their performance has only been compared in un-blinded studies. We compared the success of biphasic truncated exponential (BTE) and monophasic damped sine (MDS) shocks for defibrillation in OHCA in a prospective, randomised, double blind clinical trial. First responders were equipped with MDS and BTE automated external defibrillators (AEDs) in a random fashion. Patients in ventricular fibrillation (VF) received BTE or MDS first shocks of 200 J. The ECG was recorded for subsequent analysis continuously. The success of the first shock as a primary endpoint was removal of VF and required a return of an organized rhythm for at least two QRS complexes, with an interval of <5 s, within 1 min after the first shock. The secondary endpoint was termination of VF at 5 s. VF was the initial recorded rhythm in 120 patients in OHCA, 51 patients received BTE and 69 received MDS shocks. The success rate of 200 J first shocks was significantly higher for BTE than for MDS shocks, 35/51 (69%) and 31/69 (45%), P=0.01. In a logistic regression model the odds ratio of success for a BTE shock was 4.01 (95% CI 1.01-10.0), adjusted for baseline cardiopulmonary resuscitation, VF-amplitude and time between collapse and first shock. No difference was found with respect to the secondary endpoint, termination of VF at 5 s (RR 1.07 95% CI: 0.99-1.11) and with respect to survival to hospital discharge (RR 0.73 95% CI: 0.31-1.70). BTE-waveform AEDs provide significantly higher rates of successful defibrillation with return of an organized rhythm in OHCA than MDS waveform AEDs.