Pyonephrosis is caused by chronic obstruction of the upper urinary tract. Consequences of chronic obstruction are a functional defect of the mucosa of the ureter, diminished blood supply, and an increased interstitial fibrosis of the obstructed kidney and ascending infection. Penetrating through the transitional epithelium, bacteria come into the parenchyma of the kidney. In the region of vessel-connective tissue the immune response is disturbed. If the endotoxins of gram-negative bacteria come into the blood circulation before neutralization by humoral antibodies, urosepsis can develop.