Aztreonam was administered to 122 patients with presumptive or confirmed gram-negative bacillary meningitis in an open, multinational study. The antibiotic was administered at a dosage of 1-2 g to adults, 50 mg/kg to children greater than 2 years old, and 30 mg/kg to infants three or four times daily. Seventy-seven patients had microbiologically confirmed gram-negative meningitis due to an aztreonam-susceptible organism and received aztreonam for at least 48 hours. Haemophilus influenzae was the most frequently recovered pathogen (40 patients), followed by Enterobacteriaceae (16 patients), Neisseria meningitidis (15 patients), and Pseudomonas species (six patients). All but four patients were microbiologically cured. Microbiologic failure was associated with either a persistent intracerebral abscess (one patient) or a foreshortened course of therapy before microbiologic reevaluation and death (at 48 hours, 48 hours, and 72 hours after initiation of treatment, respectively). These data suggest that aztreonam is effective in the treatment of gram-negative bacillary meningitis caused by susceptible organisms.