Premature infants of either gender who were eligible for the natural history study had weighed less than 1,251 grams at birth and had survived the first 28 days of life. They had no major ocular or systemic congenital anomalies. Infants who met these criteria and also had a threshold level of ROP (defined as stage 3+ of the International Classification of Retinopathy of Prematurity occupying five or more contiguous or eight cumulative 30 degree sectors [clock hours] of stage 3 ROP in zone I or II in the presence of plus disease) could be referred for examination to determine eligibility for entry to the cryotherapy trial.