Type 1 diabetic patients experience symptomatic hypoglycemia once or twice a week. 30% of insulin-treated patients suffer from hypoglycemic coma at least once during their life, 10% at least once a year. 3% are incapacitated by frequent and severe attacks, 3 to 4% die from hypoglycemia. In contrast the incidence of symptomatic hypoglycemia in sulfonylurea-treated patients is about 0.2 per 1000 patients' years, the mortality of severe attacks being about 8%. The cause of hypoglycemia is always an excess of insulin, in absolute or relative terms. Additional precipitating factors are defective counterregulation after longer duration of diabetes, as well as a lowered threshold for counterregulation and/or for perception of warning symptoms, as seen under strict glycemic control. Considering the risks associated with hypoglycemia, preventive measures must be emphasized.