Purpose: To evaluate the efficacy of macular grid photocoagulation in preserving or improving central visual acuity in eyes with macular edema due to central vein occlusion (CVO) and best-corrected visual acuity of 20/50 or poorer. Methods: Patients with angiographically documented macular edema due to CVO were entered into a multicenter randomized controlled clinical trial supported by the National Eye Institute. Eligibility was determined based on both clinical examination findings and photographic documentation evaluated at a photograph reading center. Eyes were assigned randomly to macular grid photocoagulation (77 eyes) or no treatment (78 eyes). Patients were followed every 4 months for 3 years or until the end of the study. The outcome measure was visual acuity. Results: The study population consisted of 155 eyes in 155 patients. There was no difference between treated and untreated eyes in visual acuity at any point during the follow-up period. Initial median visual acuity was 20/160 in treated eyes and 20/ 125 in control eyes. Final median visual acuity was 20/200 in treated eyes and 20/160 in control eyes. However, treatment clearly reduced angiographic evidence of macular edema. Conclusion: The results of this study do not support a recommendation for macular grid photocoagulation for the population meeting the Central Vein Occlusion Study macular edema group eligibility criteria.