Optic nerve hypoplasia is frequently associated with congenital CNS anomalies and endocrine disturbances. In a prospective study of 93 children with this condition, we found that 78% of those with bilateral involvement, poor vision, and nystagmus (group 1) had additional nonocular developmental abnormalities. In contrast, among children with unilateral optic nerve hypoplasia (group 2) or with bilateral segmental hypoplasia and good vision (group 3), only 21% had nonocular developmental disturbances. Delayed development in 23 children was the most frequent nonvisual problem associated with optic nerve hypoplasia, and in five cases it occurred in the absence of detectable CNS, endocrine, or medical abnormalities. Nine patients had decreased levels of thyroid hormone; three, decreased levels of growth hormone; and three, decreased levels of both. Five had histories of neonatal hypoglycemia. Of the children with endocrine disturbances, ten had delayed development, but only seven had anomalies on computed tomographic scan.