The Branch Vein Occlusion Study is a multicenter, randomized, controlled clinical trial designed to answer several questions regarding the management of complications of branch vein occlusion. This report addresses the questions, "Can peripheral scatter argon laser photocoagulation prevent the development of neovascularization?" and "Can peripheral scatter argon laser photocoagulation prevent vitreous hemorrhage?" To answer the first question, 319 eyes were assigned randomly to either a treated or an untreated control group. Comparing treated patients with control patients (average follow-up time, 3.7 years), the development of neovascularization was significantly less in treated eyes (P = .009, log rank test). To answer the second question, 82 eyes were assigned randomly to either a treated or untreated control group. Comparing treated patients with control patients (average follow-up time, 2.8 years), the development of vitreous hemorrhage was significantly less in treated eyes (P = .005, log rank test). Although the Branch Vein Occlusion Study was not designed to determine whether peripheral scatter treatment should be applied before rather than after the development of neovascularization, data accumulated in this study suggest that peripheral scatter treatment should be applied after the development of neovascularization rather than before the development of neovascularization. Because the occurrence of vitreous hemorrhage was lessened by peripheral scatter argon laser photocoagulation, we recommend laser photocoagulation for patients with branch vein occlusion who have developed neovascularization and who meet the eligibility criteria of this study.