Hypoglycaemia is the most frequent acute complication of insulin-dependent diabetes mellitus. The clinical symptoms of insulin-induced hypoglycaemia can be grouped into those attributable to the sympathetic and adrenergic responses, e.g. tremor, pallor, palpitation, sweating, mydriasis ('hypoglycaemia awareness'), and those attributable to brain dysfunction, ranging from headache to convulsions and coma. Hypoglycaemia in diabetic children can occur at any time of the day, but nocturnal hypoglycaemia is a particular fear and worry. The frequency of mild hypoglycaemia is almost impossible to ascertain and the incidence of severe hypoglycaemia varies between 0.07 and 3.6 episodes per patient-year, though most authors report a range of 0.1-0.2 episode per patient-year. The most frequent causes of hypoglycaemia in diabetic children are deviations from treatment routine such as strenuous exercise, omission of snacks or skipped meals, and gross deviations from the prescribed times of insulin injections and recommended doses of insulin. Other predisposing factors include intensified insulin treatment, improved glycaemic control, young age, longer duration of diabetes and defective counterregulation. The available paediatric studies do not seem to support the suggestion that human insulin impairs the perception of hypoglycaemic symptoms ('hypoglycaemia unawareness') and increases the frequency of hypoglycaemic episodes, but further conclusive studies are needed. Prolonged and recurrent severe hypoglycaemia, particularly in younger children, can cause permanent neuropsychological dysfunction (e.g. learning disabilities) and permanent electroencephalographic abnormalities. Mild hypoglycaemia has also been documented to affect cognitive function, and the performance of neuropsychological tasks can remain decreased for some time (up to several hours) after full clinical recovery from hypoglycaemia. An impending hypoglycaemic attack can usually be averted by the ingestion of 20 g of rapidly absorbed carbohydrate. A severe episode can be effectively treated outside hospital with subcutaneous or intramuscular glucagon (0.5-1.0 mg) or in the hospital by an intravenous bolus of 0.2-0.5 g/kg glucose followed by a continuous glucose infusion. Patient and parent education and vigilant application of diabetes self-care principles are perhaps the most effective means of prevention, but in very young children a less strict metabolic control (higher glycosylated haemoglobin levels) may be necessary.