Pyonephrosis (from Greek pyon 'pus' and nephros 'kidney'[1]) is a dangerous kidney infection that is characterized by pus accumulation in the renal collecting system.

[3] Clinical symptoms in patients with pyonephrosis can range from frank sepsis (15%) to asymptomatic bacteriuria.

[5] Pyonephrosis can result from an upper urinary tract infection combined with blockage and hydronephrosis.

When patients do not improve quickly with appropriate antibiotics, however, appropriate radiographic studies help diagnose pyonephrosis, emphysematous pyelonephritis, and renal and/or perirenal abscesses.

[7] Together with intravenous antibiotics, drainage[8]—either percutaneous or retrograde with a ureteral stent[9]—has become the cornerstone of treatment since the development of ultrasonography and computed tomography (CT) scanning.