A multicenter, prospective clinical study of 148 culture-proven cases of bacterial keratitis, treated with ciprofloxacin 0.3% ophthalmic solution, compared its clinical and antibacterial efficacy with the standard therapy regimen of each investigator. Most often, this consisted of topically administered fortified cefazolin (33 mg/ml) and fortified gentamicin or tobramycin (14 mg/ml). Two control groups were studied: (1) patients treated while the ciprofloxacin study was ongoing (not enrolled group); and (2) patients treated during the year before initiation of the ciprofloxacin study (historical group). Treatment with ciprofloxacin yielded a 91.9% success rate. Standard therapy yielded an 88.2% success rate among not enrolled control patients and an 88.3% success rate among the historical control patients. No serious side effects accompanied ciprofloxacin administration. In vitro susceptibility testing showed ciprofloxacin to be highly effective against bacterial corneal pathogens. These findings suggest that ciprofloxacin 0.3% is safe and effective as a single agent for the treatment of bacterial keratitis.