Acute uncomplicated pyelonephritis is most commonly due to bacterial infection ascending from the lower urinary tract, and occurs more often in young adult women, 15 to 29 years old, who are sexually active; this is on account of the shorter urethra in women and its potential contamination with intestinal bacteria. As discussed in the following, complicated acute pyelonephritis has predisposing findings. The incidence is estimated to be 15 to 17 cases per 10,000 women, contrasting 3 to 4 cases per 10,000 men.