Aztreonam was administered to 20 children diagnosed as having any of the following infections: urinary tract infection, pneumonia, meningitis, and abscess of the appendix. Haemophilus influenzae, Escherichia coli, and Klebsiella pneumoniae were isolated. The minimum inhibitory concentrations of aztreonam for these bacteria ranged from 0.03 to 0.5 micrograms/ml. All patients were clinically and bacteriologically cured within 5-16 days of treatment. Six months after completion of therapy, patients who had had meningitis appeared to be free of any neurologic sequelae. The antibiotic was well tolerated by all patients.