Serial electroencephalographic recordings were made in 70 diabetic children and findings were related to age at electroencephalography and at diagnosis, duration of diabetes, daily insulin dose, long term metabolic control assessed by glycated haemoglobin A1 (HbA1) concentrations, and severe hypoglycaemic episodes. Abnormalities were found in 18 (26%) of diabetic children, and in only five (7%) of control subjects. There were no associations between electroencephalographic abnormalities and duration of diabetes, daily insulin dose, or HbA1 concentration. Diabetic children with electroencephalographic abnormalities were younger, had an earlier onset of diabetes and 21/34 (62%) of them had previously severe attacks of hypoglycaemia, whereas abnormalities were found in only 13/43 (30%) of diabetic children who had not had severe hypoglycaemia. All diabetic children with hypoglycaemic convulsions had permanent electroencephalographic abnormalities. The degree of metabolic control had no effect on the electroencephalographic findings during the early years of diabetes, but previous severe hypoglycaemia, young age, and early onset seem to be important risk factors for electroencephalographic abnormalities.