OBJECTIVE To assess the consequences of renal ultrasonography (RUS) in the treatment of children younger than 5 years of age with a first febrile urinary tract infection. METHODS We retrospectively reviewed the results of imaging studies, including RUS, computed tomography, and voiding cystourethrography in children with a first febrile urinary tract infection during a 2-year period. Children with known urologic anomalies, other underlying diseases, or simultaneous combined illnesses were excluded. Children with nephromegaly were diagnosed with acute lobar nephronia by computed tomography. RESULTS A total of 390 children were included in this study. Of the 390 children, 112 (28.7%) had abnormal RUS findings. The children with abnormal RUS findings of nephromegaly, small kidney, intermittent hydronephrosis, or a double collecting system had a significantly greater incidence of vesicoureteral reflux than children with normal RUS findings. Additionally, the occurrence of high-grade vesicoureteral reflux in children with abnormal RUS findings was more frequent than in children with normal RUS findings. CONCLUSIONS The results of our study indicate that it is worth performing RUS in children with a first febrile urinary tract infection because abnormal kidney size or other specific structural ultrasound findings should be investigated, in addition to isolated hydronephrosis.