Primary objective: We sought to assess the prevalence of shock-induced sustained return of spontaneous circulation (ROSC) and neurologically intact discharge survival before (1990-1997) and after (1998-2006) transition to biphasic waveform defibrillation in our population-controlled EMS setting. Materials and methods: All victims of out-of-hospital cardiac arrest with ventricular fibrillation as the initial rhythm from November 1990 to December 2006 were included for analysis. Data were acquired prospectively and analyzed retrospectively during two periods: before (period 1) and after (period 2) transition from monophasic damped sine to biphasic truncated exponential and rectilinear biphasic waveform defibrillation. We compared the prevalence of sustained ROSC and survival in the two periods. Results: Very high survival was observed during both periods. During period 1, sustained ROSC with shocks only was obtained in 37 (27.6%) patients; in period 2, sustained ROSC was obtained in 39 (40.2%) (p = 0.04). Fifty-three (39.6%) survived to neurologically intact discharge in period 1, and 45 (46.4%) in period 2 (p = 0.29). For bystander-witnessed arrests, ROSC was obtained in 34 (31.5%) in period 1 and 34 (45.3%) in period 2 (p = 0.06). Forty-eight (44.4%) survived in period 1 and 39 (52.0%) in period 2 (p = 0.31). We observed no other significant differences in patient or EMS-performance characteristics in the two time periods. Conclusions: Return of sustained pulses with shocks alone increased after transition to biphasic waveform defibrillation, with no other differences to explain the increase. High survival was noted in both periods, with a trend toward higher survival in the second period. © 2007 Elsevier Ireland Ltd. All rights reserved.