Background: Initial blood ammonia level is associated with neurologic outcomes in out-of-hospital cardiac arrest (OHCA). We tested the usefulness of blood ammonia for prediction of long-term neurological outcome of OHCA.Methods and Results: A total of 3,011 hospitalized adult OHCA patients were enrolled. Blood samples were obtained at the ED. Cut-offs (ammonia <100 mu mol/L and lactate <12 mmol/L) were determined in a previous study. Neurological outcomes in survivors were assessed at 3 months. A logistic regression model with adjustment for within-hospital clustering and other risk factors was used to evaluate the association between biomarkers and outcomes. Of 3,011 patients, 380 (13.8%) had favorable neurological outcomes. Ammonia and lactate predicted neurological outcome with an AUC of 0.80 (95% CI: 0.76-0.84) and 0.77 (95% CI: 0.72-0.82), respectively. Adjusted OR for ammonia <100 mu mol/L (4.55; 95% CI: 2.67-7.81) was higher than that for lactate <12 mmol/L (2.63; 95% CI: 1.61-4.28) and most other risk factors, such as cardiac etiology (3.47; 95% CI: 2.55-4.72), age <80 years (3.16; 95% CI: 2.17-4.61), bystander CPR (2.39; 95% CI: 1.70-3.38), and initial rhythm shockable (1.66; 95% CI: 1.16-2.37). The combination of ammonia and lactate had an increased predictive value (AUC, 0.86; 95% CI: 0.85-0.87) compared with that without biomarkers (AUC, 0.81; 95% CI: 0.80-0.82).Conclusions: Initial blood ammonia level is as useful as other traditional prognostic indicators such as lactate. Measurement of both initial blood ammonia and lactate helped accurately predict neurological outcomes after OHCA.