This is a study of the influence of transthoracic impedance (TTI) on defibrillation, resuscitation and survival in patients with out-of-hospital cardiac arrest (OHCA), treated with a non-escalating impedance-compensating 150 J biphasic waveform defibrillator. Cardiac arrest data from two EMS systems were analyzed retrospectively. All witnessed arrests from patients who presented with a shockable rhythm and were treated initially by BLS personnel were included (n = 102). For each defibrillation and resuscitation outcome variable, we tested differences in mean TTI for successful versus unsuccessful outcome. The effect of call-to-shock time on overall outcome was also examined. Initial shocks defibrillated 90% [83-95%] (95% confidence interval) of patients. Cumulative success with two shocks was 98% [93-100%] and with three shocks was 99% [95-100%]. TTI averaged 90 +/- 23 Omega. First-shock success, cumulative success through two shocks and cumulative success through the first-shock series were unrelated to TTI, as were BLS ROSC, pre-hospital ROSC, hospital admission and discharge. In contrast and consistent with previous findings, call-to-shock time was highly predictive of survival. High impedance patients were defibrillated by the biphasic waveform used in this study at high rates with a fixed energy of 150 J and without energy escalation. Rapid defibrillation rather than differences in patient impedance accounts for resuscitation success.