Although ischaemic heart disease is the predominant cause of mortality in older people with diabetes, age-specific mortality rates have not been published for patients with Type 1 diabetes. The Diabetes UK cohort, essentially one of patients with Type 1 diabetes, now has sufficient follow-up to report all heart disease, and specifically ischaemic heart disease, mortality rates by age. A cohort of 23,751 patients with insulin-treated diabetes, diagnosed under the age of 30 years and from throughout the United Kingdom, was identified during the period 1972 to 1993 and followed for mortality until December 2000. Age- and sex-specific heart disease mortality rates and standardised mortality ratios were calculated. There were 1437 deaths during the follow-up, 536 from cardiovascular disease, and of those, 369 from ischaemic heart disease. At all ages the ischaemic heart disease mortality rates in the cohort were higher than in the general population. Mortality rates within the cohort were similar for men and women under the age of 40. The standardised mortality ratios were higher in women than men at all ages, and in women were 44.8 (95%CI 20.5-85.0) at ages 20-29 and 41.6 (26.7-61.9) at ages 30-39. The risk of mortality from ischaemic heart disease is exceptionally high in young adult women with Type 1 diabetes, with rates similar to those in men with Type 1 diabetes under the age of 40. These observations emphasise the need to identify and treat coronary risk factors in these young patients.