Background:The effect of in-hospital rapid cooling by intravenous ice-cold fluids for comatose survivors of out-of-hospital cardiac arrest (OHCA) is unclear.Methods and Results:From the J-PULSE-HYPO study registry, data for 248 comatose survivors with return of spontaneous circulation (ROSC) who were treated with therapeutic hypothermia (34℃ for 12–72 h) after witnessed shockable OHCA were extracted. Patients were divided into 2 groups by the median collapse-to-ROSC interval (18 min), and then into 2 groups by cooling method (rapid cooling by intravenous ice-cold fluids vs. standard cooling). The primary endpoint was favorable neurological outcome (Cerebral Performance Category of 1 or 2) at 30 days after OHCA. In the whole cohort, the shorter collapse-to-ROSC interval group had significantly higher favorable neurological outcome than the longer collapse-to-ROSC interval group (78.2% vs. 46.8%, P<0.001). In the shorter collapse-to-ROSC interval group, no significant difference was observed in favorable neurological outcome between the 2 cooling groups (rapid cooling group: 79.4% vs. standard cooling group: 77.0%, P=0.75). In the longer collapse-to-ROSC interval group, however, favorable neurological outcome was significant higher in the rapid cooling group than in the standard cooling group (60.7% vs. 33.3%, P<0.01) and the adjusted odds ratio after rapid cooling was 3.069 (95% confidence interval 1.423–6.616, P=0.004).Conclusions:In-hospital rapid cooling by intravenous ice-cold fluids improved neurologically intact survival in comatose survivors whose collapse-to-ROSC interval was delayed over 18 min after shockable OHCA.