To present a case of severe hydronephrosis, left demonstrated by antenatal sonography in a 1-month-old male infant whotook prophylactic antibiotics with amoxicillin for one month since the 3rd day after his birth. Fever occurred after discontinuingthe prophylactic antibiotics for 2 days while no vesicoureteral refl ex (VUR) was confi rmed. Urosepsis withEscherichia coli was later found. Ureteropelvic junction obstruction (UPJO), left with grade 4 hydronephrosis was diagnosedafter performing percutaneous nephrostomy and antegrade pyelography. Then, dismembered pyeloplasty was doneto cleave the ureteropelvic junction obstruction. The treatment in infants with UPJO, including the options of prophylacticantibiotics and indication of surgery, is discussed in this essay.