A review of the paediatric and adolescent literature over the past 15 yrs reveals inconsistent findings concerning the relative neurotoxicity of hyperglycaemia and hypoglycaemia (9, 10, 23–25, 31–33, 38–46)–see Table 1. Earlier reports tended to support the role of hypoglycaemia in explaining neurocognitive deficits in youth with type 1 diabetes. The possible contribution of hyperglycaemia was either not considered, in the belief that youth are yet to experience the cerebrovascular complications evident in adults with long duration diabetes, or was tested using a very limited sampling of glycosylated haemoglobin (HbA_1c) levels. Recent studies, using improved methodology, report positive associations between hyperglycaemia and neurocognitive deficit in youth with diabetes (10, 23, 31, 41, 46), with hypoglycaemia also implicated in some (10, 23, 46), but not all, of these reports.