The initial treatment of the urinary tract infections (UTI) is empirical and it is a priority to determine the antibiotic resistance of most common germs in a population. Furthermore, due to the suspicion of acute pyelonephritis the presence of renal scarring should be ruled out as this may lead to further complications. A retrospective longitudinal study was performed on all children under 14 years diagnosed with UTI from January 1 2009 to December 31 2009. The in vitro susceptibility to the most important urinary pathogens was analysed, along with the presence of scars, and a subsequent follow-up. The most frequently isolated bacteria were E. coli (80%), P. mirabilis (9.7%) and K. pneumoniae (4.2%). In the antibiogram, E coli showed a high sensitivity to fosfomycin (99.1%), cefotaxime (98.2%) cefuroxime (97.3%) and gentamicin (95.6%). The sensitivity obtained against amoxicillin-clavulanate was 83.2%, while that obtained against cotrimoxazole was 78.9%. Post-pyelonephritis scars were found in 19% of patients with febrile UTI, 17% out-patients and 20% of those admitted., (Copyright © 2011 Asociación Española de Pediatría. Published by Elsevier Espana. All rights reserved.)