The incidence of acute pyelonephritis is higher in young women than in men but the incidence in men over 65 years of age is similar to that in older women. Women have up to a 10% risk of recurrent acute pyelonephritis in the year following a first acute episode, the equivalent risk in men is lower at 6%. Acute pyelonephritis may be uncomplicated and resolve without serious sequelae. However, a minority of episodes may be complicated by acute kidney injury, papillary necrosis, renal or perinephric abscess or the development of emphysematous pyelonephritis.