Objective: To investigate the penetration of 0.5% moxifloxacin hydrochloride into the aqueous and vitreous after topical administration in humans. Methods: A prospective, nonrandomized study of 20 patients scheduled for vitrectomy surgery between September 1 and December 31, 2003. Aqueous and vitreous samples were obtained and analyzed after topical administration of 0.5% moxifloxacin hydrochloride, every 2 hours (q2h) or every 6 hours (q6h), for 3 days before surgery. Assays were performed using high-performance liquid chromatography. Results: Mean± SD moxifloxacin concentrations in the q2h group for the aqueous (n=9) and vitreous (n=10) were 2.28± 1.23 and 0.11± 0.05 μ g/mL, respectively. Mean± SD moxifloxacin concentrations in the q6h group for the aqueous (n=10) and vitreous (n=9) were 0.88± 0.88 and 0.06 ± 0.06 μ g/mL, respectively. The minimum inhibitory concentration for 90% of isolates (MIC90) was far exceeded in the aqueous for a wide spectrum of key pathogens, whereas it was not exceeded in the vitreous for several organisms. However, the minimum inhibitory concentration for 50% of the isolates was exceeded in the q2h vitreous group for Staphylococcus epidermidis, Staphylococcus aureus, Streptococcus pneumoniae, Haemophilus influenzae, Bacillus cereus, and other gram-negative pathogens. Conclusions: The Endophthalmitis Vitrectomy Study revealed that 94.2% of isolates from postoperative endophthalmitis are gram-positive pathogens. Moxifloxacin has a spectrum of coverage that appropriately encompasses the most common organisms in endophthalmitis. The pharmacokinetic findings of this investigation show that relatively high aqueous levels can be achieved after topical administration. Further studies will help define the precise role of 0.5% moxifloxacin ophthalmic solution in the treatment of or prophylaxis against intraocular infections. Objective: To investigate the penetration of 0.5% moxifloxacin hydrochloride into the aqueous and vitreous after topical administration in humans. Methods: A prospective, nonrandomized study of 20 patients scheduled for vitrectomy surgery between September 1 and December 31, 2003. Aqueous and vitreous samples were Results: Mean ± SD moxifloxacin concentrations in in Mean ± SD moxifloxacin concentrations in the q6h group for the aqueous (n = 10) were 2.28 ± 1.23 and 0.11 ± 0.05 μg / mL, respectively. Mean ± SD moxifloxacin concentrations in the q6h group for the aqueous (n = respectively. The minimum inhibitory concentration for 90% of isolates (MIC90) was far exceeded in the aqueous for a wide spectrum of key pathogens, and respectively (n = 9) were 0.88 ± 0.88 and 0.06 ± 0.06 μg / it was not exceeded in the vitreous for several organisms. However, the minimum inhibitory concentration for 50% of the isolates was exceeded in the q2h vitreous group for Staphylococcus epidermidis, Staphylococcus aureus, Streptococcus pneumoniae, Haemophilus influenzae, Bacillus cereus, and other gram- negativelusionsgens. Conclusions: The Endophthalmitis Vitrectomy Study revealed that 94.2% of isolates from postoperative endophthalmitis are gram-positive pathogens. Moxifloxacin has a spectrum of coverage that adequatelyses the most common organisms in endophthalmitis. The pharmacokinetic findings of this investigation show that relatively high Further studies will help define the precise role of 0.5% moxifloxacin ophthalmic solution in the treatment of or prophylaxis against intraocular infections.