• We studied the clinical characteristics and hypothalamic-pituitary function in 23 patients with optic nerve hypoplasia (ONH), 6 months to 19 years old. All patients had decreased visual acuity and small optic discs; the septum pellucidum was absent in five of 19. Nine of 11 patients had minor EEG abnormalities, and two had microcephaly. The height, weight, and growth rate were normal in all patients 6 months to 3½ years old. Hypopituitarism was found in 15 patients. Fasting and stimulated prolactin levels and the area under the prolactin curve after thyrotropin releasing hormone were significantly greater than in controls and in patients with idiopathic hypopituitarism. These results associate ONH with a high incidence of hypopituitarism, hyperprolactinemia, and neurologic abnormalities. The normal growth in the absence of measurable growth hormone suggests that hyperprolactinemia may stimulate growth temporarily and that a normal height in childhood or the presence of the septum pellucidum do not exclude the possibility of hypopituitarism.