The purpose of this study is to evaluate the effect of a single preoperative injection of intravitreal bevacizumab (IVB) on visual outcome in patients undergoing pars plana vitrectomy (PPV) for proliferative diabetic retinopathy (PDR). Retrospective chart review of patients who underwent PPV for PDR and followed for at least 6 months after surgery. Patients who received a single IVB injection (1.25 mg in 0.05 ml) preoperatively were assigned to group A. Those who did not receive IVB were assigned to group B. The primary outcome measure was visual outcome at 6-month follow-up. The secondary outcomes were postoperative complications. At 6 months, visual acuity improved by at least 0.3 logMAR units in 70 (74.5 %) of the 94 eyes in group A versus 46 (52.9 %) of the 87 eyes in group B (p = 0.002). Postoperative vitreous hemorrhage occurred significantly less frequently in group A (6 eyes, 6.4 %) than in group B (14 eyes, 16.1 %; p = 0.037). The incidence of other postoperative complications did not differ significantly between the two groups. Preoperative use of bevacizumab improved the 6-month visual outcome in patients undergoing PPV for PDR and significantly reduced the occurrence of postoperative vitreous hemorrhage.