The incidence of hypoglycaemic episodes in patients with type 1 diabetes mellitus (DM1) is frequent. They experience 1-2 symptomatic hypoglycaemias per week. 10-20 % of the patients suffer from at least one severe hypoglycaemia per year. The incidence of severe hypoglycaemia in a group of intensely treated patients is about three times as high as compared with standard treatment and 55 % of all episodes occur during sleep. Prevention of hypoglycaemia, and restoration of euglycaemia resp. include disappearance of metabolic effects of insulin and activation of glucose contraregulating systems among which there is a certain hierarchy. The phenomenon of unaware hypoglycaemia (FNH) is defined as failure to diagnose autonomous warning symptoms, and their non-appearance before the development of neuroglycopenia. The incidence of FNH in the population with DM1 is frequent. Based on a standardized insulin diffusion test 26 % of patients with DM1 suffer from it which means that every fourth patient is affected. In the pathogenesis of FNH various factors and mechanisms were suggested as predisposing. Most probably a defect at the level of the central nervous system is involved caused by: 1. a reduced ability to recognize the decline of the blood sugar level by the CNS (altered function of the hypothalamic glucostat and/or glucose transport across the haematoencephalic barrier), 2. reduced secretion of neurotransmitters, 3. reduced tissue response to adequte neurotransmitter secretion. The theory of etiopathogenesis must explain and define its association with the peristence of diabetes mellitus, strict metabolic control, autonomous neuropathy and repeated episodes of hypopglycaemia. The contemporary hypothesis of the pathogenesis of FNH is the mechanism of repeated frequent hypoglycaemia which leads to general adapative changes at the level of the CNS by increased glucose transport across the haematoencephalic barrier which leads to reduced hormonal responses and reduction of symptoms. Thus a dangerous circulus vitiosus is created where hypoglycaemia induces unawareness of hypoglycaemia. This condition is at least partly reversible. The presence of FNH should influence the decision of the physician before using an intensified insulin regimen in diabetics.

        Key words: Diabetes mellitus - Hypopglycaemia - Phenomenon of unawareness of hypoglycaemia