Ninety-three percent of eyes that develop neovascular glaucoma (NVG) following central retinal vein obstruction (CRVO) have an ischemic index greater than 50%. An ischemic index (percentage of retinal capillary nonperfusion) of 50% represents approximately 10 disc areas of retinal ischemia as determined by computer analysis of standard 30 degrees fluorescein angiograms. The difficulties of following patients clinically and angiographically at frequent intervals over extended periods of time, and the tendency for iris neovascularization (NVI) to develop and to progress rapidly to NVG with painful loss of vision emphasizes the importance of early recognition and treatment of high-risk eyes. In this prospective study (1976--81), 100 consecutive eyes with an ischemic CRVO pattern (average ischemic index 82%) received early argon laser panretinal photocoagulation (PRP) and none developed NVG unless another ischemic event occurred following treatment. Prophylactic PRP in high-risk ischemic CRVO eyes appears to eliminate virtually the devastating complications of NVG.