The Branch Vein Occlusion Study is a multi-center, randomized, controlled clinical trial designed to answer several questions regarding the management of complications of branch vein occlusion. This report discusses the question, "Is argon laser photocoagulation useful in improving visual acuity in eyes with branch vein occlusion and macular edema reducing vision to 20/40 or worse?" One hundred thirty-nine eligible eyes were assigned randomly to either a treated or an untreated control group. Comparing treated patients to control patients (mean follow-up 3.1 years for all study eyes), the gain of at least two lines of visual acuity from baseline maintained for two consecutive visits was significantly greater in treated eyes (P = .00049, logrank test). Because of this improvement in visual acuity with argon laser photocoagulation of macular edema from branch vein occlusion, we recommend laser photocoagulation for patients with macular edema associated with branch vein occlusion who meet the eligibility criteria of this study.