Inclusion Criteria:  Children above 3 months old and under 18 years old Consultation in a participating pediatric emergency department Suspicion of UTI with fever (Fever ≥ 38°C and urine dipstick test positive for leukocytes and/or nitrites) First episode of UTI with fever Written informed consent of the holders of parental authority Affiliated to Social Security  Exclusion Criteria:  Criteria of severity:  Severe infection with severe sepsis or septic shock Dehydration ≥ 10% Fever ≥ 38°C > 4 days (96h) Indication of surgical or interventional drainage  Complication risk factors:  Any anatomic or functional defect of the urinary tract Pregnancy Severely immunocompromised patient Severe chronic renal failure defined as a clearance < 30 mL/min/1.73 m2 Severe liver failure 3GC allergy  Contra-indication to SXT:  G6PD deficiency Treatment with methotrexate Allergy to sulfonamide Antibiotic treatment within 48h before admission Empirical antibiotic treatment not recommended