Urinary tract infection (UTI) may predominantly involve the lower urinary tract, i.e. acute cystitis, or upper urinary tract consisting of the renal pelvis and kidney,, i.e. acute pyelonephritis The incidence of acute pyelonephritis is higher in young women than in men but the incidence in men over 65 is similar to that in older women. Women have up to a 10% risk of recurrent acute pyelonephritis in the year following a first acute episode. The equivalent risk in men is 6%. Acute pyelonephritis may be uncomplicated and resolve without serious sequelae. A minority of episodes may be complicated by acute kidney injury, papillary necrosis, renal or perinephric abscess or the development of emphysematous pyelonephritis. Acute pyelonephritis is generally caused by microorganisms ascending from the urethra via the bladder into the upper urinary tract. Rarely the kidney may be seeded by blood-borne infection. Ecoli is the most common uropathogen causing pyelonephritis accounting for 70-90% of infections. Species of Enterococci, Klebsiella, Pseudomonas, Proteus and Staphylococci are responsible for the remaining infections. There is a rising incidence in the community of UTI with bacteria that produce extended spectrum beta-lactamase (ESBL) enzymes. These ESBL bacteria have developed resistance to antibiotics such as penicillin, cephalosporins and increasingly to quinolones. Risk factors for uncomplicated acute pyelonephritis include recent sexual intercourse, acute cystitis, stress incontinence and diabetes and for complicated acute pyelonephritis include pregnancy, diabetes, anatomical abnormalities of the urinary tract and renal calculi.