Hydronephrosis due to congenital ureteropelvic junction obstruction (UPJO) is commonly diagnosed by antenatal sonography. We report the case of an infant who developed new-onset hydronephrosis in the right kidney following uneventful left-sided pyeloplasty for a congenital UPJO. The furosemide minus 15-minute diuretic renogram (F-15 DR) was used to confirm obstruction in the affected renal unit when standard diuretic renography was equivocal. The indications for surgery or observation as well as the role of the F-15 DR are discussed.