OBJECTIVE: To assess the possibility of heart rate variability (HRV) measures as predictors of 24-h mortality in successfully resuscitated patients with out-of-hospital cardiac arrest (OHCA). METHODS: This prospective cohort study was conducted at a 40-bed emergency department (ED) of a university-affiliated medical centre. Adult patients with OHCA who were successfully resuscitated were consecutively enrolled over an 18-month period. A 10-min electrocardiogram was recorded for retrospective off-line HRV analysis 30-60 min after the return of spontaneous circulation and further correlated with 24-h mortality of the patients. RESULTS: Sixty-nine patients aged 31-82 years who met the inclusion criteria were enrolled. According to the 24-h mortality, the patients were categorised into non-survivors (n=28) and survivors (n=41) groups. The HRV measures were compared between these two groups. The low-frequency power (LFP), normalized LFP (nLFP) and low-/high-frequency power ratio in the non-survivors were significantly lower than those of the survivors, whereas root mean square successive difference, high-frequency power (HFP), HFP/tidal volume, normalized HFP (nHFP), and nHFP/tidal volume in the non-survivors were significantly higher than those of the survivors. Multiple logistic regression model identified nLFP as the independent variable to predict 24-h mortality (odds ratio, 1.354; 95% confidence interval [CI], 1.124-1.632; p=0.001). Receiver operating characteristic area for nLFP in the prediction of 24-h mortality was 0.946 (95% CI, 0.897-0.995; p<0.001). CONCLUSIONS: HRV measures, especially the nLFP, may be used as predictors of 24-h mortality for successfully resuscitated patients with OHCA in the ED.