Acute pyelonephritis is frequent. Its usual signs and symptoms comprise renal pain, fever, inflammation, and pyuria. Simple acute E. coli pyelonephritis is frequent in the young female and in most cases is a benign condition. A typical pyelonephritis may be painless, or without high fever, or lacking bacterial growth in the urine due to previous inappropriate treatment. Severe, complicated pyelonephritis is mainly observed in diabetic, alcoholic or immunocompromised patients. In occasional cases, a common form of pyelonephritis may progress to formation of a renal abscess requiring drainage. When secondary to urinary tract abnormalities, pyelonephritis may be complicated with septicemia and can induce early and severe renal tissue damage. This form warrants emergency urologic treatment. Simple pyelonephritis of the young female without febrile UTI history requires little imaging. Conversely, extensive imaging workup is mandatory in the male, the elderly, when treatment is not rapidly effective or in case of early relapse. In some cases, acute pyelonephritis leads to the development of cortical scars, the long-term prognosis of which remains to be determined.