What is the difference between chronic pyelonephritis and hydronephrosis?

Infected/obstructed. Pyelonephritis refers to a kidney infection, whereas hydronephrosis refers to obstruction of the kidney leading to dilation and swelling. You should see your doctor and possibly a urologist for both of these issues to get them evaluated to see if an anatomic abnormality or stone is contributing to infection or to see cause/severity of obstruction and if corrective surgery required.