Acute focal bacterial nephritis (AFBN) is a localized bacterial infection of the kidney presenting as an inflammatory mass without frank abscess formation, which may represent a relatively early stage of renal abscess. The pathogenesis of AFBN is thought to be hematogenous infection or ascending infection from the lower urinary tract. For the majority of children, the pathogenesis may be related to ascending infection, because pre-existing malformative uropathy, especially vesicoureteral reflux (VUR), is common in pediatric patients. Few clinical reports have demonstrated the evolution of this condition into renal abscess in children. We report a girl with AFBN associated with VUR that evolved into a renal abscess despite appropriate intravenous antibiotic therapy. We undertook serial radiological observation of its evolution. It is important to differentiate AFBN from renal abscess because the management of the two entities may be different, and follow-up studies are indispensable to determine appropriate therapy.