OBJECTIVE: Children who develop type 1 diabetes before age 7 years (early-onset diabetes; EOD) have comparatively poorer cognitive abilities. Whether this relates to psychosocial consequences of chronic illness or organic factors related to diabetes and its complications remains unresolved. We hypothesized that if differences in neuroradiological structure and cognitive ability coexisted in those who had EOD, then an organic component to their etiology was likely. RESEARCH DESIGN AND METHODS: A cohort of 71 young adults with long-duration type 1 diabetes diagnosed during childhood or adolescence participated in a cross-sectional evaluation of cognitive ability (neuropsychological test battery) and brain structure (magnetic resonance imaging). Diabetes onset age, preceding severe hypoglycemia exposure, retinopathy status, and diabetes duration were examined as potential correlates of cognitive and neuroradiological differences. No participants had previous neuropsychological pathology. RESULTS: In EOD participants (n = 26), current intellectual ability (Wechsler Adult Intelligence Scale-Revised performance IQ; P = 0.03) and information processing ability (Choice Reaction Time; P = 0.006) were comparatively poorer than was observed in those with later- onset diabetes (n = 45). Furthermore, lateral ventricular volumes were 37% greater (P = 0.002) and ventricular atrophy was more prevalent (61 vs. 20%; P = 0.01) in the EOD group than in those who had later-onset type 1 diabetes. CONCLUSIONS: An early childhood onset of type 1 diabetes was associated with mild central brain atrophy and significant differences in intellectual performance in adulthood, implying that neurodevelopment may be adversely affected by EOD. The differences observed in brain structure support an organic contribution to their etiology but do not exclude a coexistent contribution of psychosocial factors.