Urinary tract infections (UTI) may become dangerous if ascending bacteria reach the kidney and subsequently induce an inflammatory reaction within the renal parenchyma. Host factors like vesicorenal reflux or obstructive uropathies increase the risk of ascending infection and facilitate renal damage even by bacteria of "low virulence". Inadequate antibiotic therapy and young age are further predisposing factors for pyelonephritic scars. Hypertension, reduced kidney function and complications during pregnancy are the potential long term problems arising from renal damage. Early recognition of obstructive uropathies by sonographic screening, undelayed diagnosis and consequent treatment of symptomatic UTI, and early identification of reflux after symptomatic UTI's may reduce the incidence of these problems.