Objective: To investigate long-term mortality rates and causes of death in individuals diagnosed with type 1 diabetes before the age of 15 years during the period 1989-2012 or known to paediatric diabetes teams in 1989, in Northern Ireland.Methods: A cohort of 3129 patients from the Northern Ireland Childhood Diabetes Register was linked to death registrations and underlying causes, coded according to ICD-9 or ICD-10.Standardized mortality ratios (SMRs) were calculated as the ratio of observed to expected deaths by sex, attained age, time since diagnosis, calendar period, and cause of death.Results: Subjects were followed to December 31, 2012 giving 39 764 person-years of follow-up (median 12.1 years). In total, 59 subjects had died (1.5 per 1000 person-years) compared with 19.9 deaths expected, an SMR of 296 (95% confidence interval (CI) 229-382).Women had a significantly higher excess risk of mortality than men with SMRs of 535 (95% CI 361-764) and 203 (95% CI 136-291), respectively. Over half of the deaths (56%) were judged to be related or possibly related to diabetes with most of these due to acute (n = 24) or late (n = 6) complications.Conclusions: Subjects with type 1 diabetes diagnosed less than 15 years of age had 3 times the mortality risk of the general population. Over half of the deaths were related to acute or chronic complications of diabetes. K E Y W O R D Smortality, standardized mortality ratio, type 1 diabetes