The monobactam aztreonam was used to treat 22 young patients with meningitis caused by gram-negative bacilli. Haemophilus influenzae was isolated from the CSF of 21 patients and Salmonellaheidelberg from the CSF of 1. Dosages ranged from 100 to 200 mg/kg/day in 4 doses at 6-hour intervals. Minimal inhibitory concentrations were determined by the broth dilution method for all isolated strains, and values ranged from 0.05 to 2.0 μg/ml. Blood and CSF drug levels were determined by a microbiologic plate diffusion method, and mean values for CSF and blood were 1.4 and 14.9 μg/ml, respectively. The outcome was good in 21 patients; 1 patient died. Complications were mild: subdural effusion occurred in 6 cases and was managed clinically; asymptomatic hydrocephalus was seen in 4; seizure during the acute phase occurred in 6 cases; hypo-acusis was noted in 2, and motor impairment was detected at the follow-up in 1 case. Aztreonam achieved good blood and CSF penetration and performed well in the treatment of 20 cases of H. influenzae meningitis and in the one case of S. heidelberg meningiti