From January 2007 to June 2007, consecutive patients undergoing primary pars plana vitrectomy with silicone oil infusion for complications of proliferative diabetic retinopathy will be recruited for the prospective study. The selection criteria are: 1) anticoagulant therapy has not been used prior to surgery or during post-operative follow-up period; 2) no medical history of blood diseases associated with abnormal blood coagulation is present. Active PDR is defined as visible large new vessels within the proliferative tissue with fresh preretinal and/or vitreous hemorrhage. Decision of silicone oil infusion will be made before surgery when potential creation of multiple breaks or incomplete traction release is anticipated during surgery due to severe vitreoretinal adhesion. The morphological criteria set for silicone oil infusion are: severe active fibrovascular proliferation with broad vitreous attachment around the disc, arcade and extending to the periphery in at least 2 quadrants (≥grade 5 in Eliott's grading system);8 presence of macular-off traction or combined traction and rhegmatogenous retinal detachment.