Altitudinal visual field defects were discovered in four young women on routine eye or general physical examinations. This led to extensive investigations because three of them were falsely presumed to have acquired disorders of the central nervous system. In each case the defect appeared to result from congenital hypoplasia of the optic nerve associated with maternal diabetes mellitus. Congenital optic nerve hypoplasia should be included in the differential diagnosis of visual field defects even when they are discovered in adults. Hypoplasia consequent to maternal diabetes mellitus is apt to be associated with excellent visual acuity and is usually an isolated defect. The female offspring of diabetic mothers may be at greater risk than the male offspring for the development of this anomaly.