PDR is a leading cause of irreversible vision loss in North America. This disease is caused by the growth of abnormal blood vessels in the retina. These abnormal blood vessels can bleed inside the eye, causing a vitreous hemorrhage (VH). Sometimes when patients have this bleeding, a surgery called vitrectomy is required to remove the blood from within the eye. In order to reduce complications during the surgery, most retina surgeons will inject Avastin into the eye a few days before the surgery.

Avastin (bevacizumab) is currently not approved by Health Canada to treat any ocular disease. Lucentis (ranibizumab) is approved by Health Canada as a treatment for age-related macular degeneration, diabetic macular edema, and retinal venous occlusive disease. While Avastin is not approved by Health Canada for the treatment of these diseases, the majority of retina specialists around the world are now using Avastin "off-label" to treat these diseases. That is because Avastin and Lucentis both tend to work equally well in these disease, but Avastin is significantly cheaper. While Avastin and Lucentis are generally regarded to be equal, there may be some differences between these two drugs that have not been discovered. The aim of this study is to look for these differences.

Previous research by the investigators in this study has shown that injecting Avastin into eyes causes increased inflammatory proteins to develop inside the eye. This increase in these proteins was related to complications that developed after the vitrectomy surgery. Lucentis may be associated with less of an increase in inflammatory proteins (and less complications). The aim of this study will be to compare Avastin and Lucentis with respect to how they affect inflammatory proteins in the eye, as well as the rate of complications during surgery.

Study participants will be divided into two arms ("groups") of 30 subjects. Subjects will receive Avastin or Lucentis a few days before vitrectomy surgery. The assignment will be random and the study is double-masked. Masking is done so that the investigators can clearly determine any differences between the 2 drugs.

60 subjects will take part in this study at 2 sites in Canada: Vancouver (Eye Care Centre, Vancouver General Hospital, and Mount Saint Joseph Hospital) and Toronto (Sunnybrook Health Sciences Centre).

Inclusion Criteria:

1. PDR and vitreous hemorrhage scheduled for vitrectomy surgery and bevacizumab pre-treatment.

Exclusion criteria:

1. Vitreous hemorrhage from other causes such as central retinal vein occlusion or ocular ischemic syndrome.
2. Pregnant or breastfeeding women.
3. Less than 19 years of age.

In order to assess changes in intraocular levels of cytokines following anti-VEGF treatment, the ideal time to measure these cytokines is at baseline and approximately 1 week later. In patients with PDR who are scheduled for vitrectomy, many vitreoretinal surgeons now inject bevacizumab approximately 1 week prior to the surgery in order to decrease the risk of intra-operative complications. These patients are thus an excellent study group for studying intraocular cytokine changes in response to anti-VEGF therapy, since that are already scheduled to have 2 intraocular procedures performed approximately 1 week apart, thereby minimizing the risk of obtaining aqueous humour.