Hypoglycaemia unawareness, a frequent syndrome in insulin-dependent diabetes mellitus (IDDM), involves a decrease or absence of perception of specific symptoms which normally inform the subject that plasma glucose is decreasing to dangerous levels leading to neuroglycopenia. Without warning symptoms, IDDM patients are unable to take measures (e.g. eating) to prevent severe neuroglycopenia (unconsciousness). As hypoglycaemia unawareness is associated with impaired glucose counterregulation, especially reduced adrenaline responses, it can lead to severe hypoglycaemia. Various studies in animals and humans have indicated that hypoglycaemia unawareness is largely, if not entirely, secondary to increased brain glucose transport due to recurrent or chronic hypoglycaemia. In fact, meticulous prevention of hypoglycaemia largely restores the warning symptoms and adrenaline responses, at least in short-term IDDM. In long-term IDDM, recovery is less complete. Diabetologists and IDDM patients need to be familiar with hypoglycaemia unawareness and how to prevent or treat it. Intensive therapy strictly for normoglycaemia may actually increase the frequency of hypoglycaemia and hypoglycaemia unawareness. However, if intensive therapy is combined with a hypoglycaemia prevention programme, the percentage of HbA1c can be maintained below risk values for the onset or progression of complications, and the frequency of hypoglycaemia can be kept low. Under these conditions, IDDM patients can maintain the warning symptoms and adrenaline response to hypoglycaemia, ensuring a vital backup system for safe intensive therapy of IDDM.