Acute pyelonephritis is one of the most common serious bacterial infections in childhood, particularly in young children, with an estimated prevalence in febrile infants of 5.3%. It has been considered an important risk factor for the development of renal scarring, and renal insufficiency. More recently the long term outcomes of pyelonephritis has ameliorated, probably because of prompt diagnosis and therapy, and the importance of urinary tract infection as a risk factor for renal insufficiency has been questioned.