Microvascular disease, manifest as retinopathy, neuropathy or nephropathy, frequently complicates diabetes, the risk being related to long-term glucose control and increasing disease duration. Microvascular disease may also affect the cerebral circulation and could potentially compromise brain structure and intellectual performance. Type 1 diabetes commonly develops in childhood before maturation of the central nervous system and the developing brain may exhibit relative vulnerability to damage as a consequence of exposure to severe hypoglycaemia, or the development of Diabetic Keto-Acidosis, in early childhood. Genetic factors influence the vulnerability of an individual to develop cognitive impairment following pathological processes known to disadvantage the central nervous system. Polymorphism of the Apolipoprotein-E gene has been identified as one such factor and is known to influence the prognosis and cognitive outcomes following a wide variety of cerebral results. The studies contained within this Thesis explore the long-term consequences the clinical factors described above on brain structure and the cognitive performance of young adults with Type 1 diabetes mellitus of long duration. The effects of polymorphism of the Apolipoprotein-E gene on the cognitive performance of young adults who have Type 1 diabetes mellitus are also evaluated.