To compare the usefulness of shock index (SI) and lactate for prediction of 24-hour and 28-day mortality in acute heart failure (AHF) patients. A descriptive study. Place and Duration of the Study:Balikesir University Hospital, Balikesir, Turkey; from February 2019 to August 2020. One hundred and twelve AHF patients presenting to ED were recruited into the study. Usefulness of lactate and SI in predicting mortality at 24-hour and 28-day, was evaluated. The area under the curve (AUC) was found to be 0.825 for lactate and 0.818 for SI in predicting 24-hour mortality. There was significant difference between the diagnostic performances of 2 markers in predicting 24-hour mortality. AUC was found to be 0.775 for lactate and 0.722 for SI in predicting 28-day mortality. No significant difference was found between the diagnostic performances of the two markers in predicting 28-day mortality. The 24-hour non-survivor rates were found to be 86.67% in patients with lactate levels >2.57; 76.47% in patients with SI >0.94, and 93.33% in patients with lactate levels >2.57 or SI >0.94. The 28-day non-survivor rates were found to be 64.71% in patients with lactate levels >2.57; 70.59% in patients with SI >0.82, and 82.35% in patients with lactate levels >2.57 or SI >0.82. Lactate level and SI of AHF patients calculated in ED triage may be used to predict mortality, and simultaneous use of both parameters may be more helpful. Key Words: Acute heart failure, Emergency department, Lactate, Mortality, Shock index.