Background The Acute Heart Failure Index (AHFI) is a previously derived prediction rule to identify patients presenting to emergency departments (EDs) with decompensated heart failure (DHF) at low risk for early life-threatening events. Study Objectives We sought to prospectively validate the AHFI. Methods Using a prospective cohort study, we included adult patients presenting to an urban university hospital ED with DHF. We gathered data on 21 variables to calculate the AHFI. Primary endpoints included inpatient death and non-fatal serious outcomes (myocardial infarction, ventricular fibrillation, cardiogenic shock, cardiac arrest, intubation, or cardiac reperfusion). Secondary endpoints included death from any cause or readmission for heart failure within 30 days. We calculated primary and secondary endpoint rates with 95% confidence intervals (CI) for the low- and higher-risk subgroups. Results We enrolled 259 patients. 245/259 (95%) were admitted. 60/259 (23%) met low risk criteria, of whom 1/60 (1.7%, CI 0.04–8.9) was discharged after sustaining pulseless electrical activity arrest. The comparable primary outcome rate in the derivation study was 1.4% (CI 1.1–1.7). 17/199 (8.5%, CI 5.1–13.3) higher-risk patients experienced an endpoint, compared with 13.3% (CI 12.9–13.7) in the derivation cohort. 1 low-risk patient (1.7%, CI 0.04–8.9) died within 30 days, and 5 (8.3%, CI 2.8–18.4) were readmitted. Corresponding rates in the derivation study were 2% and 5% respectively. Conclusion Our results are consistent with those previously reported for the low-risk subgroup of the AHFI. Further research is needed to determine impact, safety and the full range of generalizability of the AHFI as an adjunct to decision making.