Mortality and the causes of death have been studied in a population-based cohort of 4919 childhood onset IDDM cases. Enrolment began in 1977 and at the time of study there had been a maximum duration of disease of 13.5 years, with a total of 33,721 person years at risk. Survival status was ascertained by linkage to the Swedish Cause-of-Death register. Death certificates, autopsy protocols, and hospital records were scrutinized for classification of causes of deaths. Twenty males and 13 females with IDDM died before the age of 28.5 years. This corresponds to a Standardized Mortality Rate for age of 262% (95% confidence limits, 172-400) for the boys and 384% (95% confidence limits, 232-635) for girls. Seven patients died of ketoacidosis, four at onset of diabetes. Nine cases were found 'dead in bed', having been seen apparently healthy 1-2 days before death. One of these cases had signs of cerebral haemorrhages at autopsy and another one had signs of bite marks in the mouth, but otherwise all autopsies were normal and no evidence of alcohol or other intoxication was found. In a well-educated population with good access to inexpensive diabetes care, there is still a two- to threefold excess mortality among young onset insulin-dependent diabetic individuals including a high frequency of unexplained deaths in bed.