Perinephric abscess is a life-threatening but treatable process. Most infections of the perinephric space occur as a result of extension of an ascending urinary tract infection, commonly in association with nephrolithiasis or urinary tract obstruction. A large portion of the mortality is the result of failure to diagnose this entity in a timely fashion. This failure may be because of the frequently obscure or nonspecific nature of the clinical presentation. Blood cultures as well as urine cultures may fail to identify correctly the bacterial pathogens responsible for the abscess. Perinephric abscess should be considered in the differential diagnosis of any patient presenting with a urinary tract infection that fails to respond promptly to antibiotic therapy, particularly in those known to have anatomical abnormalities of the urinary tract or diabetes mellitus. Consideration of this diagnosis should enter into the differential diagnosis of fever with abdominal pain or flank pain. Early recognition of perinephric abscess and prompt drainage, either percutaneously or surgically, in combination with appropriate antibiotic coverage, should reduce dramatically the morbidity and mortality from this infection.