RETROLENTAL fibroplasia is a name applied by Terry1 in 1942 to a mass of vascular embryonic connective and retinal tissues observed behind the lens in eyes of infants of premature birth. Before Terry's time many sporadic cases of fibrosis behind the lens were reported. A great variety of descriptive terms were used, such as vascular cord in the vitreous, pseudoglioma, persistent hyaloid, atypical vitreous, persistent vitreous, fibrosis of the lens, microphthalmos, embryonic fibrous sheath of the lens and congenital retinal folds. There has been no increase in the sporadic forms of retrolental fibrosis. The past decade has shown a remarkable increase in what has been called "retrolental fibroplasia" in infants of premature birth. This increase is proportional to the degree of prematurity and the lowness of the birth weight, and it is selective in its geographic locality. The large group of infants and children with this defect composes about