Preoperative administration of bevacizumab (Avastin; Genentech, Inc, South San Francisco, California, USA), a full-length recombinant humanized monoclonal antibody targeting vascular endothelial growth factor (VEGF), has been reported in prospective clinical trials to decrease the overall surgery time, lower the rate of intra-operative complications, and reduce the occurrence of postoperative hemorrhaging in PDR patients with active neovascularization and/or extensive fibrovascular proliferation undergoing PPV. Furthermore, two meta-analysis studies examining published randomized controlled trials support the use of intravitreal bevacizumab (IVB) as a preoperative adjunct. Although IVB is widely used as a preoperative adjunct in patients with PDR undergoing PPV, little clinical data is available regarding the optimal timing of preoperative IVB administration or the most effective dose. In this randomized clinical study, we attempt to elucidate the most appropriate interval and dose for the administration of preoperative IVB in patients with PDR undergoing PPV for non-clearing vitreous hemorrhaging, TRD or extensive fibrovascular proliferation.