The diagnosis of urinary tract infections is based on the number of colony-forming units that grow on the culture media. The criteria for the diagnosis depends on the method of collection: clean void and bags > 10(5)/mL; transurethral catheterization > 10(4)/mL. The diagnosis of acute pyelonephritis or cystitis is based on clinical and biological signs: unexplained fever, abdominal pains, signs of neonatal infections, elevated white blood cell count, C-reactive or sedimentation rate. After an urinary tract infection, it is important to look for a lithiasis, an obstructive uropathy, a vesico-ureteral reflux and a bladder dysfunction. The child should have a sonogram and a voiding cystourethrography. In case of cystitis the antimicrobial therapy is given orally. In case of pyelonephritis, most of the time the treatment is initially administered parenterally for 3 days and then completed by a 10-day antimicrobial course orally. In a child over 18 months of age who is not toxic appearing without severe uropathy, the antimicrobial therapy can be initiated orally.