OBJECTIVE To investigate the intraocular penetration of moxifloxacin into the aqueous and vitreous after oral administration in humans. METHODS A prospective, nonrandomized study of 27 consecutive patients scheduled for elective parsplana vitrectomy surgery between 1 October and 31 December 2004 was carried out. Aqueous, vitreous, and serum samples were obtained and analysed after oral administration of a single 400 mg tablet of moxifloxacin a few hours before surgery. Assays were performed using high-performance liquid chromatography. RESULTS Mean+/-SD moxifloxacin concentrations in the serum (n=27), aqueous (n=25), and vitreous (n=27) were 1.34+/-0.98, 0.21+/-0.21, and 0.09+/-0.09 microg/ml, respectively. The mean+/-SD sampling times after oral administration of the moxifloxacin tablet for serum, aqueous, and vitreous were 2.02+/-0.51, 1.53+/-0.45, and 1.55+/-0.46 h, 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. Of note, the MIC90 for Staphylococcus epidermidis was not exceeded in any of the samples. CONCLUSIONS Orally administered moxifloxacin achieves measurable levels in the noninflammed human eye, with the aqueous levels effective against a variety of pathogens. However, the spectrum of coverage does not appropriately encompass the most common causative organisms in endophthalmitis, especially Staphylococcus epidermidis. Further studies are needed to precisely define the role of oral moxifloxacin in the treatment of or prophylaxis against intraocular infections.