The incidence of unexplained deaths ('dead in bed syndrome') in Norwegian diabetic patients under the age of 40 was investigated during the period 1981-1990. During this 10-year period there were 240 deaths of all causes in the age group 0-39 years. Sixteen of these cases fulfilled the following criteria of the 'dead in bed syndrome': (1) patient found dead in an undisturbed bed; (2) patient observed to be in good health condition the day before; (3) no clinical evidence of late complications (except background retinopathy in two cases). Of the 16 cases ascertained, 10 were males and 6 females. The age range was 7-35 years, and the duration of diabetes varied between a few months and 26 years. Autopsy, performed in 13 cases, did not reveal any cause of death. Nine patients had been using insulin regimens with multiple daily doses. Twelve patients were reported as having had frequent episodes of hypoglycaemia, with nocturnal episodes in 10 cases. There was apparently an increasing incidence of unexplained deaths during the study period, with 12 of 16 cases occurring in the years 1988 to 1990.