Hydronephrosis is the dilation of the renal collecting system (pelvis and calyces) resulting from obstruction to urine outflow or, less commonly, vesicoureteral reflux. On ultrasound it appears as an anechoic fluid-filled space replacing the central renal sinus echoes. Severity is graded from mild (mild pelvic dilation only) to severe (marked calyceal and pelvic dilation with cortical thinning). Common causes include ureteral calculi, ureteropelvic junction obstruction, and extrinsic compression from tumor or lymphadenopathy.