BACKGROUND: Urinary tract infections (UTIs) are commonly treated in the emergency department (ED), and unfortunately, resistance to first-line agents is increasing. OBJECTIVES: To characterize treatment of pyelonephritis in a nationally representative sample of ED patients and to identify patient- and treatment-specific factors associated with receiving initial inactive antibiotics. METHODS: We conducted a multicentre, observational cohort study utilizing the Emergency Medicine PHARMacotherapy Research NETwork (EMPHARM-NET), comprising 15 geographically diverse US EDs. All patients ≥ 18 years of age with a diagnosis of pyelonephritis between 2018 and 2020 were included. The primary endpoint was the proportion of patients who received initial inactive empirical antibiotic therapy and to identify predictive factors of inactive antibiotic therapy. RESULTS: Of the 3714 patients evaluated, 223 had culture-positive pyelonephritis. Median patient age was 50.1 years and patients were mostly female (78.3%). Overall, 40.4% of patients received an IV antibiotic, most commonly ceftriaxone (86.7%). The most frequently prescribed antibiotics were cefalexin (31.8%), ciprofloxacin (14.3%), cefdinir (13.5%) and trimethoprim/sulfamethoxazole (12.6%). Overall, 10.3% of patients received initial inactive therapy. After adjustment in a multivariable analysis, long-acting IV antibiotic was predictive of inactive therapy (OR 0.23, 95% CI 0.07-0.83). CONCLUSIONS: In our prospective, multicentre observational study, we found that only 40.4% of patients with pyelonephritis received empirical IV antibiotics in the ED, contributing to inactive therapy. Receipt of long-acting IV antibiotics was independently associated with a decreased rate of initial inactive therapy. This reinforces guideline recommendations to administer long-acting IV antibiotics empirically in the ED upon suspicion of pyelonephritis.