Deficiencies in the release of glucagon and adrenaline during hypoglycaemia in diabetic patients are associated with a high frequency of severe hypoglycaemic episodes. These have been attributed to a resulting inability to increase hepatic glucose output acutely. A more important consequence may be reduced awareness of impending hypoglycaemia. Such hypoglycaemia unawareness is related to, but not entirely explained by, diminished sympathetic activity, perhaps due to failure to activate the sympathoadrenal system until a lower blood glucose level is achieved. The patient's subsequent failure to act appropriately would then be due to impaired cerebral function preventing the perception of sympathoadrenal activation during the more severe hypoglycaemia. Unawareness of moderate hypoglycaemia is common in diabetic patients. Autonomic neuropathy probably accounts for only a few cases of hypoglycaemia unawareness and other factors which are potentially important include duration of diabetes, glycaemic control, insulin species, and the degree and frequency of hypoglycaemia. Further research is required to discover both the pathophysiological mechanisms of, and the treatment needed to reverse or prevent, the abnormality.