We evaluate in this retrospective cohort, the clinical situations leading emergency physicians to take a blood lactate sample, the prevalence of hyperlactatemia and its impact on short-term adverse outcome. ED patients requiring a blood lactate measurement (BLM) during a two-year period were included. Early patients' outcomes were extracted and discharge diagnoses were classified into 12 diagnostic categories. A total of 118,737 patients were analyzed. A BLM was carried out in 13,089 of them. Surprisingly, the proportion of patients having a BLM was higher in those admitted for seizure (31.4%) than in those admitted for infection (27.9%). Ten percent of patients who had a blood lactate test had a lactate level >4 mmol/l (1,315). Among them, 23.2% were admitted for infections, 20% for seizures, and 11% for cardiovascular diseases. After excluding the patients older than 75 years from the analysis in order to prevent a selection bias, the patient's severity was independently associated to an age over 65 years (OR: 1.26), an arterial blood sampling (OR: 2.77) and the blood lactate level (OR: 1.31). The blood lactate level was very informative to detect the sicker patients in the infection group whereas its interest was poor in the group of patients admitted for seizures. In conclusion, blood lactate testing has become routine in emergency departments and a large proportion of patients have abnormal blood lactate levels. The most frequent causes of high blood lactate in the ED are infection and seizures but the prognostic value of blood lactate seems to be different from one diagnostic category to the other.