Acute focal bacterial nephritis (AFBN), formerly known as lobar nephronia, is a rare form of interstitial bacterial nephritis. Most often described in adults with diabetes, there is only limited knowledge of AFBN in children. Ultrasound shows circular hypoechogenic, hypoperfused parenchyma lesions, which may be misdiagnosed as a renal abscess or tumor. From 1984 to 2005, AFBN was diagnosed in 30 children at the University Hospital Münster and the General Hospital Celle, Germany. Data of 25 cases (14 girls, 11 boys) were available for retrospective evaluation. Twenty-five children with AFBN, mean age 4.5 years (range: 0.25-17.5 years), were followed up on average 4.2 years (range: 0.5-11 years). All children were admitted to hospital due to fever and rapid deterioration of clinical condition, initially suspected of having meningitis (four patients), urinary tract infections (five patients), renal tumor (three patients), pneumonia (two patients), appendicitis (one patient), or with only unspecific symptoms (ten patients). AFBN was diagnosed by ultrasound on average 3 days (range: 1-10 days) after onset of symptoms. Pyuria was found in 18/25 children, bacteriuria in 20/25 children, and hematuria in one patient. Blood cultures were negative in all but one patient. Urinary tract abnormalities were found in 12 children, including vesicoureteral reflux (8), megaureter (1), urethral valves (1), unilateral renal hypoplasia (1), and one patient with megacystis, megaureter, caudal dystopic left kidney combined with hypoplasia and dysplasia of the right kidney. High-resolution ultrasound showed AFBN lesions to have resolved completely within 12 weeks after onset of intravenous antibiotic therapy in 20/25 children. Renal parenchymal cysts remained in three cases and focal scarring in two. Blood pressure and renal function was normal in 24/25 cases. AFBN should be suspected in children with fever and rapid deterioration of clinical condition. Residual lesions such as cysts or scarring of renal parenchyma could remain.