Pyonephrosis is pus in the kidney. Pus is an inflammatory exudate that is rich in neutrophils and cellular debris.

This is a specimen of a kidney cut in longitudinal section. The hilus of the kidney is in the bottom left corner of the screen. Note that in this slide most of the pus has been removed from the calyces. Pyonephrosis is often the result of an infection secondary to urinary outlet obstruction, e.g. benign prostatic hypertrophy. The stagnant urine in the kidney is an excellent medium for the growth of bacteria. The treatment is both surgical and medical. Note that pyonephrosis is different from pyelonephritis.