Escherichia coli is the most frequent cause of pyelonephritis. A possible virulence factor includes the ability to adhere to and colonize the urinary tract which is an important initiating factor in all urinary tract infections. The importance of P-fimbriae and type 1 fimbriae in this adhesion and inflammatory process are emphasized and evidence for their importance in pyelonephritis is presented in epidemiologic studies of patients, as well as in animal studies. It is widely accepted that uropathogens possess definite determinants of virulence which enable them to survive and multiply in vivo. Among these pathogen,E. coli has the capability of provoking tissue damage and scar formation in the parenchyma, with scarring the major pathologic feature of chronic pyelonephritis. The mechanisms involved in renal scarring are still poorly understood, but some investigators have noted that renal scarring is related to an acute inflammatory process due to the respiratory burst of phagocytosis and renal ischemia as a consequence of the release of superoxide. Vaccination with P-fimbriae has been shown to experimentally prevent the initiation of the disease. However, since vaccines are not clinically available, clinical and animal studies on the therapy of acute disease are presented. The prevention of end-stage renal disease that may occur from acute pyelonephritis during infancy depends on early diagnosis, age, and effective antibiotic treatment.