Pyonephrosis (Greek pyon "pus" + nephros "kidney") is an infection of the kidneys' collecting system. Pus collects in the renal pelvis and causes distension of the kidney. It can cause kidney failure.
Pyonephrosis is sometimes a complication of kidney stones, which can be a source of persisting infection. It may also occur spontaneously. It can occur as a complication of hydronephrosis or pyelonephritis.