Type 1 and type 2 diabetes are associated with major complications that shorten life expectancy. Fluctuations in the amount of glucose reaching the brain cause a wide range of alterations to cerebral function, from mild impairment through to dementia-like states and even death. The responsibility is on the patient with diabetes to control his or her blood glucose levels to reduce any mental decline. However, nursing staff should be aware that patients with diabetes experiencing a loss of mental function may have difficulty in understanding information and instructions. This article examines the current literature on loss of mental function as a chronic complication of diabetes.