Antimicrobial susceptibility of 400 consecutive positive bacterial isolates from urine cultures (≥ 105 colony counts per ml) of a similar number of ambulatory (n: 216) and hospital (n: 184) male and female patients (300 girls) under age 15 year, from downtown Santiago, Chile, is described. All samples were processed at a university associated hospital of the chilean public health services system. Bacterial sensitivity was tested by disk diffusion (Kirby-Bauer). Escherichia coli was by far the most frequently isolated pathogenic bacteria (75.7% of the whole, 89.1% outpatient girls, 58.5% outpatient boys, 76.8% inpatient females and 45.7% inpatient males). Klebsiella pneumoniae was second in outpatients girls (3.4%), and Proteus mirabilis in boys (22.0%). Among inpatients Klebsiella pneumoniae, plus Klebsiella sp were second (13.4% in girls and 18.5% in males). Pseudomonas aeruginosa was only identified from inpatient males (10.1%). One third of E. coli comming from ambulatory patients were resistant to first generation cephalosporins, more than 50% were not sensitive to ampicillin and trimethoprim-sulfamethoxazole, but 96% or more were susceptible to gentamicin and a similar proportion to nitrofurantoin (≥ 96%). In samples obtained from hospital patients, 28% E. coli were resistant to the ampicillin-sulbactam association (p 0.0000053), more than 40% to first generation cephalosporins, over 50% to trimethoprim-sulfamethoxazole and a similar percentage to ampicillin, while 95% or more were susceptible to amikacin and gentamicin (p 0.000). K. pneumoniae were almost always sensitive to amikacin (93.7%) but gentamicin was effective in only 20% of them (p 0.000148). Almost one half K. pneumoniae were sensitive to nitrofurantoin as well. Six in seven cases of P. aeruginosa were susceptible to ceftazidime. Unless local susceptibility studies show different results it looks reasonable to suggest that, in this community, initial treatment of patients under age 15 year with lower urinary tract infections should include serious consideration to nitrofurantoin as a first line drug. The same seems to be true for amikacin, while susceptibility tests are worked up, when upper urinary infections are suspected among these patients.