MATERIALS AND METHODSPatients. The study population consisted of 30 patients, 16 males and 14 females, with a mean age of 4.2 years (range, 0.03 to 15.4 years). Underlying urological abnormalities (12 cases of hydronephrosis, 9 of grade III to IV vesico-ureteral reflux, 3 of uretero-pelvic junction obstruction, 2 of spastic neurogenic bladder, and 5 cases of other multiple abnormalities of the urinary tract) were present in 21 patients of whom 10 had surgical treatment for their urological abnormalities in the previous 2 weeks. One patient had renal calculosis. According to the normal values for age given by Goldsmith (7), 13 patients had renal failure, mild in 6, moderate in 5, and severe in 2 (creatinine clearance, 50 to 80, 30 to 50, and <30 ml/min per 1.73 m2, respectively). In patients <13 years old, glomerular filtration rate was estimated from plasma creatinine concentration and body length by using two different formulas for patients who were <1 year and >1 year old (24,25). Seven patients had been previously treated for the same infection in another hospital with appropriate doses (one case each) of co-trimoxazole, ampicillin, cefotaxime, piperacillin, gentamicin, amikacin, and netilmicin. These antibiotics were reported to be initially active in vitro against the infecting organisms. We discontinued them after at least 6 days because of microbiological failure (bacteriuria > 105 CFU/ml and the development in 6 of 7 cases of in vitro resistance to the drug as determined by Kirby-Bauer criteria). Before aztreonam treatment, seven other patients received co-trimoxazole and six received ampicillin as prophylactic antibiotics; in these cases, also, the pathogens were 310