Acute pyelonephritis is a bacterial infection of the kidney parenchyma that can cause organ damage and be life-threatening. It is associated with systemic symptoms and flank pain which are not usually present in lower urinary tract infections. Early recognition and aggressive management of sepsis is required to prevent serious complications. Patients should be urgently referred to the emergency department if they are unwell and showing signs of sepsis (hypotension, tachycardia, hypothermia or pyrexia, signs of systemic illness such as rigors). Patients at higher risk of severe disease (elderly comorbid patients or those in other risk groups) may also warrant urgent referral.