For infants weighing less than 1,251 grams at birth, the Cryotherapy for Retinopathy of Prematurity (CRYO-ROP) Study has shown that the risk of developing severe, acute (threshold) ROP is 6 percent. Although cryotherapy, when applied at the time of threshold ROP, reduces the rate of unfavorable visual outcome, 35 percent of eyes that develop this level of severe, acute disease are blind 1 year after treatment. Moreover, cryotherapy is destructive. Even when cryotherapy prevents progression to retinal detachment, it is associated with peripheral retinal destruction and may, in some cases, be associated with subnormal central vision due to high myopia and/or macular scarring. Corrective surgical treatments for retinal detachment caused by ROP have proven to be of little visual benefit. A preventive treatment for ROP that is safe, efficacious, easily applied, and inexpensive is desirable.