Gram-negative bacillary meningitis (typically caused by E. coli, K. pneumoniae, P. aeruginosa, A. calcoaceticus, and others) has been associated with head trauma, neurosurgical operations, pathologic anastomosis with nasal cavity, and CSF rhinorrhea, as well as with impaired host defenses. Intravenous ciprofloxacin was given to 20 patients with gram-negative bacillary meningitis. The dose of ciprofloxacin was 200 mg by intravenous infusion, over 30 minutes, every 12 hours for 10 days. Two patients also received intravenous cefotaxime and penicillin G. All patients were monitored daily. Of 20 patients, 18 had cure and therapy failed in two. Because relatively low concentrations of ciprofloxacin are achieved in the CSF, the drug should be used in the treatment of gram-negative bacillary meningitis only if the MICs of the causative pathogen and the ciprofloxacin concentration in CSF can be followed. A randomized, controlled study is needed to compare the efficacy of intravenous ciprofloxacin with standard antibiotic therapy in the treatment of this disease.