Mild therapeutic hypothermia has proved beneficial after out-of-hospital cardiac arrest in the adult population, when the initial rhythm is ventricular fibrillation (VF). In this study, data from 110 consecutive patients with out-of-hospital cardiac arrest due to VF (n = 86) or to non-VF rhythm (n = 24), admitted to an intensive cardiac care unit with restoration of spontaneous circulation and who remained unconscious on admission, were analyzed. Patients were cooled using an external cooling system. Of the patients with VF, 66% had favorable outcomes (Glasgow-Pittsburgh Cerebral Performance Category 1 or 2), and 30% died. Of the patients with non-VF, 8% had favorable outcomes (p <0.001 vs VF), and 63% died (p = 0.004 vs VF). In patients with VF, those with poor outcomes were older than those with favorable outcomes (odds ratio [OR] 1.61, 95% confidence interval [CI] 1.03 to 2.7, p = 0.001) and had previous ejection fractions <35% (OR 7.72, 95% CI 1.8 to 33, p = 0.002). Outcomes were also worse when patients presented to the emergency room with seizures (OR 20.96, 95% CI 2.48 to 177.42, p = 0.003) or hemodynamic instability (OR 14.4, 95% CI 3.47 to 60, p <0.0001). In the non-VF group, the 2 patients with good outcomes were younger than those with unfavorable outcomes (39 ± 16 vs 65 ± 12 years, respectively, p = 0.04), with good left ventricular function on presentation (100% vs 4.5%, p = 0.0001) and with short asystole and/or short time from collapse to restoration of spontaneous circulation. In conclusion, mild therapeutic hypothermia in the adult population is more effective in patients with VF compared to those with non-VF. Good prognostic factors for patients with non-VF could be young age, good left ventricular function, and short anoxic time.