We previously showed urinary activity against Escherichia coli both after oral amoxicillin-sulbactam and sulbactam alone. A study was performed in 10 laboratories from 8 countries to determine the etiology of community acquired lower urinary tract infections (CALUTI) in patients aged 3-to 7 years in South America. Each laboratory sent data from 100 consecutive isolates including susceptibility to amoxicillin and amoxicillin-sulbactam by agar diffusion and also sent 20 consecutive E. coli isolates resistant (R) to amoxicillin-sulbactam to CEA reference laboratory, were MICs were determined for amoxicillin, amoxicillin-sulbactam (2/1) and sulbactam. Urines from 12 volunteers who received a single oral dose 500/500 mg amoxicillin-sulbactam were collected at 0-2; 2-4 and 4-6 h and urinary inhibitory titers (UIT) were determined against 5 R and 1 susceptible (S) isolates from each center. Levels of amoxicillin and sulbactam were determined. Results: Etiology: E. coli 820/1000 (82%) was prevalent; P. mirabilis and K. pneumoniae (4,3% each), S. saprophyticus 4,1% other 5,3%. Diffusion test: 59,4% E. coli were R to amoxicillin, 16,9 intermediate (I) and 23,7 S whereas results for amoxicillin-sulbactam were 28% R; 19,2 I and 52,8 S. MICs: 102 E. coli R to amoxicillin-sulbactam were studied; MIC90s (µg/ml) were the following: amoxicillin > 2.048; amoxicillin-sulbactam 256/128 and sulbactam alone 128. UITs: ranged from > 1/32 at 2 h dose; 1/16-1/4 h and 1/4-1/2 at 6 h against all the isolates. Antibacterial urine levels (µg/ml) obtained for amoxicillin and sulbactam respectively were at 2 h 1.414 and 1.904; at 4 h 691 and 1.257 and at 6 h 462 and 641. Our results confirm the prevalence of E. coli and explain discrepancies between resistance shown by agar diffusion test and clinical success observed in the treatment of CALUTI.