A prospective study of symptomatic hypoglycaemia was conducted in 47 children over a 14-week period using a questionnaire completed at home for each episode of hypoglycaemia. Twenty-nine children (62%) experienced 150 episodes during the study. The average incidence was once every 33 days (range 0-5.2 mo-1). Hypoglycaemia occurred more frequently in children with lowest haemoglobin A1 levels. Episodes were not randomly distributed in time; hypoglycaemia occurred significantly more frequently in the evening, in the early morning and around midday. The majority of episodes were judged to be mild but 2 children had nocturnal convulsions and glucagon was used on three occasions. Symptomatic nocturnal hypoglycaemia occurred one or more times in 30% of the children. Daytime episodes were manifested by tremor, feeling weak, dizziness, pallor, and other symptoms and signs. In 46% of cases the cause was not evident to parents or children, but 25% were related to physical activity.