Multicystic dysplastic kidneys and diagnostic confusion on renal scan.

The combination of ultrasound and renal scan usually provides a reliable means of differentiating congenital ureteropelvic junction obstruction with hydronephrosis from multicystic dysplasia in the newborn with an abdominal mass. Rarely, a multicystic kidney will show definite function on a renal scan. We describe 2 newborns who were thought to have hydronephrosis based on a renal scan but who had typical multicystic dysplastic kidneys at operation.