Pyelonephritis is inflammation of the kidney, typically due to a bacterial infection. Symptoms most often include fever and flank tenderness. Other symptoms may include nausea, burning with urination, and frequent urination. Complications may include pus around the kidney, sepsis, or kidney failure.

It is typically due to a bacterial infection, most commonly Escherichia coli. Risk factors include sexual intercourse, prior urinary tract infections, diabetes, structural problems of the urinary tract, and spermicide use. The mechanism of infection is usually spread up the urinary tract. Less often infection occurs through the bloodstream. Diagnosis is typically based on symptoms and supported by urinalysis. If there is no improvement with treatment, medical imaging may be recommended.

Pyelonephritis may be preventable by urination after sex and drinking sufficient fluids. Once present it is generally treated with antibiotics, such as ciprofloxacin or ceftriaxone. Those with severe disease may require treatment in hospital. In those with certain structural problems of the urinary tract or kidney stones, surgery may be required.

Pyelonephritis affects about 1 to 2 per 1,000 women each year and just under 0.5 per 1,000 males. Young adult females are most often affected, followed by the very young and old. With treatment, outcomes are generally good in young adults. Among people over the age of 65 the risk of death is about 40%, though this depends on the health of the elderly person, the precise organism involved, and how quickly they can get care through a provider or in hospital.