The renal pelvis is the funnel-shaped central collecting space within the kidney where urine drains from the calyces before flowing into the ureter. On ultrasound, the renal pelvis is part of the echogenic central sinus complex and is typically not distended when the patient is well hydrated and has no obstruction. When the renal pelvis becomes dilated — a condition called hydronephrosis — it appears as an anechoic, fluid-filled space within the central sinus, and the severity is graded from mild to severe based on the degree of dilation. Dilation of the renal pelvis can result from obstruction (stones, tumors, strictures), vesicoureteral reflux, or a normal variant such as an extrarenal pelvis. Measurement of the anterior-posterior diameter of the renal pelvis is particularly important in prenatal and pediatric ultrasound for screening for urinary tract abnormalities.