The suggestion of an increase in the number of sudden deaths of young people with Type 1 diabetes in the UK has been investigated. It was suggested that such deaths were due to hypoglycaemia and related to the increasing use of human insulin. In total we were notified of 50 deaths of people with Type 1 diabetes under age 50 years in the UK in 1989 which our informants (relatives, physicians, and pathologists) considered sudden and unexpected. An autopsy had been done in all cases and we supplemented this with detailed clinical information from relatives and case records. Of the 50 cases we excluded five with a definite cause of death, 11 suicides or self-poisonings, six cases of ketoacidosis, and four in which there was insufficient information about the circumstances of death to drawn any conclusions. Of the other 24 cases, two patients had been found with irreversible hypoglycaemic brain damage and died after a period of artificial ventilation. The most puzzling group were 22, aged 12-43 years, most of whom had gone to bed in apparently good health and been found dead in the morning. Nineteen of the 22 were sleeping alone at the time of death and 20 were found lying in an undisturbed bed. Most had uncomplicated diabetes and in none were anatomical lesions found at autopsy. There are major difficulties in diagnosing hypoglycaemia post-mortem, but the timing of death and other circumstantial evidence suggests that hypoglycaemia or a hypoglycaemia-associated event was responsible. All patients were taking human insulin at the time of death but most had been changed from animal insulin between 6 months and 2 years earlier and there was nothing to implicate the species of insulin as a factor in these deaths.