Bacterial infections of the urinary tract (UTI) cause a spectrum of clinical conditions. They are commonly diagnosed into three groups: acute pyelonephritis, acute cystitis and asymptomatic bacteriuria (ABU). Acute cystitis accounts for most of the UTI associated morbidity. The clinical routines emphasize, however, the detection and treatment of acute pyelonephritis. This is due both to the severity of the acute infection, which prior to the advent of antibiotics had a mortality of 15 to 20% [1], and to the potential severe long-term effects, including destruction of the renal parenchyma [2]. Furthermore, the patients who have infections complicated by disorders of the urinary tract, or who have renal damage due to prior infections, require special attention. The criteria used to diagnose the different forms of UTI include a combination of parameters describing the localization of infection and the intensity of the host response [2–5]. Acute pyelonephritis is differentiated from other forms of UTI by symptoms and signs from tissues outside the urinary tract (loin pain, fever, malaise, increased levels of the acute-phase reactants in serum). Patients with acute cystitis typically lack these signs, but have symptoms from the lower urinary tract including dysuria and frequency. Patients with ABU have no or few symptoms, but have stable bacteriuria with the same bacterial strain in repeated cultures. The current definition of acute pyelonephritis is somewhat arbitrary since it does not include parameters which localize the infection to the kidney and/or define the extent of renal involvement. The aim of this review is to: 1.) describe methods which have been used to detect infection of the kidney; 2.) evaluate the extent of renal involvement in children with different forms of UTI, based on a comparison of the acute phase response and renal concentrating capacity; and 3.) analyze the pathogenesis of UTI, as it relates to renal infection.