Aztreonam was administered to 30 patients, ages 0.03 to 15.4 years, with severe and in 21 cases complicated urinary tract infections caused by members of the family Enterobacteriaceae and Pseudomonas aeruginosa which were resistant to ampicillin and susceptible to the study drug in vitro. A mean dose of 47.7 mg/kg was given intramuscularly every 12 h to 26 patients. In four patients with renal insufficiency, the dose was reduced according to pharmacokinetic data. Permanent urine sterilization and clinical cure were achieved in 22 patients, 13 of whom had urological malformations. In two patients with P. aeruginosa and Proteus mirabilis infections, the treatment failed. Another patient had an Escherichia coli reinfection 21 days after the end of therapy. Four patients with various urological abnormalities had gram-positive superinfections, and two patients had gram-negative superinfections during and at the end of therapy: all six had indwelling ureteric splints or pyelostomy as predisposing conditions. No adverse clinical effects were observed. Some transient and slight or moderate alterations were observed at the end of treatment: eosinophilia (nine cases), elevation of hepatic enzymes (eight cases), prolongation of prothrombin time (three cases), and neutropenia (one case). A pharmacokinetic study was performed in six patients with normal renal function and in seven patients with various degrees of renal insufficiency. The elimination half-life of the drug was inversely correlated with the glomerular filtration rate. At the dosage used, aztreonam proved effective for severe urinary tract infections caused by members of the family Enterobacteriaceae in pediatric patients.