In renal urinary tract diseases in children, there are more complications related to kidney and urinary tract malformations than tumor diseases, and urinary tract malformations are frequently diagnosed by ultrasonography in screening. It is also known that children with urinary tract infections tend to have a high rate of coexisting congenital anomalies of the kidney and urinary tract. Ultrasonography is useful for diagnosis of urinary tract malformations, such as hydronephrosis, megaureter, hypoplastic and atrophic kidney, horseshoe kidney, duplicated renal pelvis/duplicate ureter, cystic kidney disease, bladder diverticulum, and ureterocele. On the other hand, it is known that about 5% of febrile diseases are urinary tract infections in infants, and sonographic evaluation is recommended for urinary tract infections. Sonographic findings specific to urinary tract infections are difficult to obtain, and the sensitivity and specificity of ultrasound are low. Nevertheless, ultrasound is recommended for urinary tract infections, because children with congenital urinary tract malformation tend to develop renal dysfunction. By diagnosing those urinary tract malformations early, we are able to prevent chronic irreversible renal dysfunction. Examples of urinary tract infections include pyelonephritis, acute focal bacterial nephritis, renal abscess, and cystitis. In the case of pyelonephritis and acute focal bacterial nephritis, severe pyelonephritis can progress to acute focal bacterial nephritis, and it is said that further progression results in renal abscess. We should not forget nephropathy in hemolytic uremic syndrome, which is complicated by hemorrhagic enterocolitis associated with pathogenic Escherichia coli infection. For early diagnosis of these urinary tract malformations, early sonographic examination is recommended for pediatric unknown fever and/or urinary tract infections. Ultrasonography is the first imaging modality in all children suspected of any urinary tract abnormality.