Urinary tract infections (UTIs) are the leading cause of community-acquired bacterial infections in adults. They are a common reason for admission to the Emergency Department (ED), particularly when pyelonephritis is suspected. The main bacteria responsible for UTIs are Enterobacteriaceae, with Escherichia coli being the main cause, found in more than 90% of cases. The French guidelines of the SPLIF (French-Speaking Infectious Pathology Society) recommend the probabilistic use of a 3rd generation cephalosporin or a fluoroquinolone. Ceftriaxone is often chosen over cefotaxime because it can be injected only once a day, which simplifies its administration in overcrowded emergency departments. There are currently no SPLIF recommendations regarding the dosage of ceftriaxone to be administered. The IDSA (Infectious Diseases Society of America) suggests a single dosage of 1 gram/day. Ceftriaxone is a 3rd generation cephalosporin antibiotic in the β-lactam class. Its mechanism of action is based on the inhibition of bacterial cell wall synthesis. Due to its broad spectrum against Gram-positive and Gram-negative aerobic bacteria and also some anaerobic germs, ceftriaxone is a commonly prescribed antibiotic in emergency departments (Therapeutic Guidelines Limited, Melbourne, 2014; Kumar et al., 2009) because of its broad indications: neuromeningeal infections, intra-abdominal infections and urinary tract infections (UTIs). Since most UTIs requiring hospitalization do not require intensive care, the optimal dosage of ceftriaxone in this context remains to be determined. Indeed, patients in emergency departments are on average less serious, without sepsis or septic shock, and therefore with probably different pharmacokinetic parameters.