PURPOSE: Filtration failure due to excessive post-operative scarring remains a major problem after glaucoma surgery. We have investigated whether glaucoma and filtration surgery are associated with increased levels of vascular endothelial growth factor (VEGF), and whether a humanized monoclonal antibody against VEGF, bevacizumab (AvastinTM, Genentech), can reduce post-operative scar formation and improve surgical outcome. METHODS: The levels of VEGF in samples of aqueous humour were measured by ELISA. The expression of the VEGF receptors Flt-1 and KDR was analysed in cultured Tenon fibroblasts by real-time RT-PCR and western blotting. The effect of VEGF and bevacizumab on Tenon fibroblasts in vitro was determined using a proliferation assay. The in vivo effect of the antibody was investigated in a rabbit model of trabeculectomy by measuring the intraocular pressure (IOP) and bleb area, and by immunohistological analysis of angiogenesis, inflammation, and fibrosis. RESULTS: VEGF levels were increased significantly in the aqueous humour of glaucoma patients and rabbits that had undergone surgery. Both VEGF receptors were expressed on Tenon fibroblasts. Fibroblast proliferation in vitro was stimulated by delivery of VEGF, and was inhibited by administration of bevacizumab. The antibody also reduced angiogenesis and collagen deposition significantly, and improved the outcome of glaucoma surgery in rabbits. CONCLUSIONS: VEGF was up-regulated in the aqueous humour of glaucoma patients and in the rabbit model, and it stimulated fibroblast proliferation in vitro. This suggests that it is involved in the scarring processs after filtration surgery. Bevacizumab reduced the proliferation of fibroblasts in vitro and improved surgical outcome.