BACKGROUND Lactate is a frequently used biomarker in emergency departments (EDs), especially in critically ill patients. The aim of this study is to investigate the relationship between lactate and lactate clearance with in-hospital mortality in unselected ED patients. METHODS This study was carried out retrospectively in the ED of a tertiary hospital. Patients aged 18 years and older whose blood lactate level was obtained in the ED were included in the study. Patients whose lactate value did not have sufficient analytical accuracy, whose lactate value was recorded in the system 180 minutes after admission, who were admitted to the ED as cardiac arrest, and whose ED or hospital outcome was unknown were excluded from the study. According to the first measured lactate value, the patients were divided into three groups: < 2.0mmol/L, 2.0-3.9mmol/L, and ≥ 4.0mmol/L. Lactate clearance was calculated and recorded in patients with one-to-four hours between two lactate values. RESULTS During the five-year study period, a total of 1,070,406 patients were admitted to the ED, of which 114,438 (10.7%) received blood gas analysis. The median age of 81,449 patients included in the study was 58 years (IQR: 30, min: 18-max: 117) and 54.4% were female. The study found that non-trauma patients with a lactate level between 2.0-3.9mmol/L had a 2.5-times higher mortality risk, while those with a lactate level of ≥ 4.0mmol/L had a 20.8-times higher risk, compared to those with a lactate level < 2.0mmol/L. For trauma patients, the mortality risk was three-times higher for those with lactate levels between 2.0-3.9mmol/L and nine-times higher for those with a lactate level of ≥ 4.0mmol/L, compared to those with a lactate level < 2.0mmol/L. Among patients with a first measured lactate value ≥ 4.0mmol/L and a two-hour lactate clearance < 20%, the mortality rate was 19.7%. In addition, lactate, lactate clearance, and age were independent variables for mortality in this patient group. CONCLUSIONS The lactate value in unselected patients in the ED is a biomarker that can be used to predict the prognosis of the patients. In addition, lactate, lactate clearance, and age are independent predictors of mortality.