Urinary tract infections are the most common urologic disease. In adults, diagnosis of urinary tract infection is typically based on characteristic clinical features and abnormal laboratory values. Urinary tract infections are usually ascending infection from the urinary bladder. Acute pyelonephritis is the most common bacterial infection involving the kidney. The characteristic CT findings are focal or diffuse renal enlargement, hypoenhanced regions in the renal parenchyma, and impaired excretory function of the kidney. Acute pyelonephritis may progress to renal or perirenal abscess if it is not treated successfully.