ObjectivesTo describe the characteristics and clinical outcomes of children with fever ≥5 days presenting to emergency departments (EDs).DesignProspective observational study.Setting12 European EDs.PatientsConsecutive febrile children InterventionsChildren with fever ≥5 days and their risks for serious bacterial infection (SBI) were compared with children with fever Main outcome measuresSBI and other non-infectious serious illness.Results3778/35 705 (10.6%) of febrile children had fever ≥5 days. Incidence of SBI in children with fever ≥5 days was higher than in those with fever 0.90, but were observed infrequently (range: 0.4%–17%). Absence of warning signs was not sufficiently reliable to rule out SBI (sensitivity 0.92 (95% CI 0.87–0.95), negative likelihood ratio (LR) 0.34 (0.22–0.54)). CRP ConclusionChildren with prolonged fever have a higher risk of SBI, warranting a careful clinical assessment and diagnostic workup. Warning signs of SBI occurred infrequently but, if present, increased the likelihood of SBI. Although rare, clinicians should consider important non-infectious causes of prolonged fever.