Renal scars have been thought to occur only in later stages of chronic pyelonephritis. In our experimental pyelonephritis model, bacteria with mannose-sensitive (MS) pili on its surface promoted renal scarring when inoculated into renal parenchyma. Pretreatment with recombinant human granulocyte-colony-stimulating factor (rhGCSF) inhibited the renal scarring which followed inoculation with MS-piHated bacteria, whereas posttreatment at an early stage of infection had no effect on renal scarring. These findings suggest that rhGCSF may be useful for the prevention of infection without increasing the tissue damage to the renal parenchyma which leads to the renal scarring. Even when rhGCSF is used for treatment of kidney infection, it does not promote increased renal scarring through the increased invasion of leukocytes at the inflammatory site.