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While serial renal ultrasound is often used as an alternative to functional renal imaging in children followed for hydronephrosis, it is unclear whether a lack of hydronephrosis progression safeguards against loss of renal function. In this study we characterize the association between findings on serial renal ultrasound and diuretic renography in children with severe unilateral hydronephrosis.We retrospectively reviewed imaging among patients younger than 18 years old with a history of severe unilateral hydronephrosis, 2 renal ultrasounds and 2 diuretic renograms. Each pair of renal ultrasounds was interpreted by an independent blinded diagnostic radiologist and compared to a contemporaneous diuretic renogram. Change in hydronephrosis was considered as 1) a change in hydronephrosis grade or 2) any change by radiologist interpretation. A 5% or greater change in split differential function was considered significant. Chi-square and Spearman correlation analyses were performed.A total of 85 children were evaluated. Increased hydronephrosis was noted in 11.8% of children by grade and 32.9% by radiologist interpretation. Split differential renal function worsened by 5% or more in 17.6% of children. Overall, 13.3% of children with stable or decreased hydronephrosis demonstrated worsening split differential function at an average of 11.8 months. When renal ultrasound and diuretic renograms were directly compared, the Spearman correlation was poor (r = 0.24, 95% CI 0.03 to 0.43).The overall correlation between imaging modalities was poor, and 13.3% of children with stable or decreased hydronephrosis had worsening of split differential renal function. These findings are important to consider when counseling nonoperatively managed children followed without diuretic renography.