Background/aims-To study the intravitreal antibiotic concentrations and the eYcacy of an intravitreal dosing regimen to treat patients with postoperative bacterial endophthalmitis. This regimen, based on pharmacokinetic/pharmacodynamic considerations, relies on a repeat antibiotic injection of a lower dose than is generally used. Methods-In consecutive patients with suspected postoperative endophthalmitis a vitreous biopsy for bacterial culture was taken before 0.2 mg vancomycin and 0.05 mg gentamicin were injected intravitreally. After 3 or 4 days a second biopsy was taken for bacteriological culture and to measure intravitreal vancomycin and gentamicin concentrations, followed by a repeat injection of 0.2 mg vancomycin. Results-17 patients entered the study. In 11 patients the initial bacterial culture was positive, predominantly coagulase negative staphylococci. All second vitreous biopsies were sterile. Intravitreal vancomycin levels varied between 2.6 and 18.0 µg/ml (mean 10.3 (SD 4.1) µg/ml) after 3 days and between 3.1 and 16.6 µg/ml (mean 7.5 (6.2) µg/ml) after 4 days which is well above the minimal inhibitory concentration for most micro-organisms. Concentrations of intravitreal gentamicin varied between 0.90 and 3.3 µg/ml (mean 1.6 (0.72) µg/ml) after 3 days and between 1.2 and 2.6 µg/ml (mean 1.9 (0.99) µg/ml) after 4 days. Conclusion-This dosing regimen resulted both in adequate intravitreal vancomycin and gentamicin levels for over a week as well as in negative second cultures. This study also provides new information on intravitreal vancomycin and gentamicin concentration over time in patients with postoperative endophthalmitis. (Br J Ophthalmol 2001;85:1289-1293 Bacterial endophthalmitis is a severe complication of intraocular surgery that may rapidly cause permanent visual loss. Therefore, when confronted with a possible postoperative infection of the eye, timely diagnostic and empirical therapeutic decisions should be made by medical history taking and a focused examination without the aid of the results of laboratory and culture results. If it is likely that the symptoms and signs are due to bacterial infection, prompt antimicrobial treatment is indicated.In empirical therapy, the activity spectrum of the antibiotic should cover all likely causative micro-organisms. Other important parameters directing the choice and dosing regimen of antibiotics in endophthalmitis include the ability to achieve eVective concentrations of the drug inside the eye, especially the vitreous, and the toxicity of the drug for retinal structures. The vast majority of postoperative eye infections are caused by Gram positive bacteria such as coagulase negative staphylococci, S aureus, and streptococci.1-3 Awaiting culture results and susceptibility testing, the methicillin resistance of many strains of staphylococci make glycopeptides such as vancomycin the antibiotic of choice. 1 3-6 As the penetration of this drug into non-vitrectomised, inflamed eyes after intravenous administration is unreliable, direct intr...