Bacterial, especially endogenous bacteria such as Escherichia coli, Proteus and Klebsiella.
Most cases are due to ascending infections from urine flow abnormalities.
Most cases are related to pre-existing urine outflow abnormalities such as tumor, pregnancy, calculi, prostatic hypertrophy, neurogenic bladder and vesicoureteral reflux. Women are more prone than men, possibly due to differences in urethral anatomy. Renal parenchymal involvement tends to be regional, not universal, commonly upper and lower poles. Cortical surfaces show suppurative abscesses. Since many cases are associated with urine outflow obstruction, dilatation of pelvis and calyces may be present.