To establish whether impaired hypoglycaemic awareness is associated with increased rate of severe hypoglycaemia and to assess clinical predictors of severe episodes without warning symptoms a prospective study of 130 insulin-dependent diabetic children and adolescents was undertaken for 1 year. Using a structured questionnaire, 48 patients reported impaired awareness and 82 reported normal awareness of hypoglycaemia at baseline of the study. The two groups did not differ regarding clinical and metabolic characteristics. Episodes of severe hypoglycaemia were recorded for 1 year. The rate of severe hypoglycaemia was higher in the group with impaired awareness than in the group with normal awareness (p < 0.0001). Of the severe hypoglycaemic episodes, 34.0% developed without warning symptoms. Patients with impaired awareness experienced more severe episodes without warning symptoms than those with normal awareness (p = 0.0054). Severe hypoglycaemia occurred more frequently in patients with impaired awareness aged 6 years and less (p = 0.0041) than in older counterparts. Impaired awareness reported at baseline [adjusted odds ratio (OR): 5.8; p =0.0021], age 6 years or less (3.4; p = 0.0121), previous severe episode (4.8; p = 0.0043) and more than 5 % of home blood glucose readings 3.3 mmol/l or less in the preceding month (4.2; p = 0.0211) proved to be independently predictive of severe hypoglycaemic events without warning symptoms. In conclusion, impaired hypoglycaemic awareness is associated with an increased rate of severe hypoglycaemia in diabetic children and adolescents. One third of severe episodes developed without warning symptoms. Impaired awareness, young age and recent biochemical or severe hypoglycaemias are independent risk factors for such episodes. Avoidance of hypoglycaemia should be a priority in preschool children with diabetes.