Purpose: To determine the prevalence of and risk factors for superior segmental optic nerve hypoplasia in offspring of mothers with type I diabetes mellitus. Methods: Thirty-four subjects aged between 4 and 37 years, the children of 23 mothers who had type I diabetes mellitus at the time of pregnancy, aged between 4 and 37 years, were recruited from one institution and prospectively examined in search of an optic disk–related anomaly. Results: Of the 34 subjects, three females (8.8%) showed classic ophthalmoscopic and perimetric features of superior segmental optic nerve hypoplasia, bilateral in two patients and unilateral in one. Pregnancies leading to affected children showed a tendency to be shorter, birth weight to be lower, and control of maternal diabetes mellitus to be poorer compared with pregnancies resulting in unaffected children. No variable unique to the affected subjects as opposed to the unaffected majority could be identified. Conclusions: We found a superior segmental optic nerve hypoplasia, described as a “topless disk,” in three of 34 subjects (8.8%) at risk for this condition. Topless disk thus seems to be more common than was previously thought, possibly having been missed because of its subtle signs and only mild impairment of visual performance in affected individuals. Female sex, short gestation time, low birth weight, and poor maternal diabetes control may represent additional risk factors for the development of a topless disk. Its pathogenesis remains obscure, but the responsible pathogenic event may occur in the perinatal period.