Background Long-term outcomes in young people with type 1 diabetes continue to be of interest, and may help evaluate the effects of changes to the clinical care of children that have occurred in recent decades.

Aims To identify mortality and its causes before age 30 years in patients developing type 1 diabetes before age 15 years.

Methods Since 1995, paediatricians in Wales have compiled a prospective register of incident cases of type 1 diabetes occurring before age 15 years in Wales (the Brecon Cohort). Their subsequent mortality rates were compared with mortality in the general populations of Wales and England using the patient-years exposure method. Causes of death were ascertained from death certificates and from clinicians.

Results The standardised mortality ratio for young people with type 1 diabetes in Wales was 2.91 with no clear evidence of improvement or worsening of mortality risk over time. Most deaths occurred between ages 15 and 30 years although at a slightly younger age than in the general population. There were more deaths with increasing age at diagnosis of diabetes. Ketoacidosis remains the most common cause of death before age 30 years. Hypoglycaemia was difficult to ascertain with certainty but also caused some deaths. In this age group, chronic complications of diabetes were not a cause of mortality.

Conclusions Despite the developments in clinical care in recent years, the mortality risk for people developing type 1 diabetes in childhood remains high in young adult life before the onset of chronic complications.