The thinning of renal cortex is usually caused by hydronephrosis, especially a large amount of hydronephrosis, which can compress the renal parenchyma and make the kidney ischemic and necrotic, thus causing the thinning of renal parenchyma. There are two causes of hydronephrosis: first, congenital stenosis. The most common congenital stenosis in clinical practice is pelvic and ureteral junction stenosis, which is caused by congenital lesions, resulting in poor elasticity of the ureteral and pelvic junction. As age increases, the stenosis will become heavier and heavier, leading to more and more fluid accumulation, which will affect the kidney function and lead to thinning of the renal cortex. Second, acquired obstruction, including scarring caused by inflammation or ischemia, can cause ureteral stenosis, and renal pelvis and ureteral stones and tumors can also cause hydronephrosis, which can affect the thinning of the kidney cortex in severe cases. Prostate enlargement, bladder neck contracture, urethral stricture, bladder stones, and encopresis can also lead to upper urinary tract emptying obstruction and formation of hydronephrosis, which can also lead to thinning of the renal cortex in severe cases. So when there is thinning of the renal cortex, it is recommended to have a urographic examination of the urinary system to see what is causing it.