BACKGROUND Diabetes is one of the most common chronic diseases in Western populations. There have been few large published cohort studies of diabetes with 20 years of follow-up worldwide, and none other than the present one in NZ. OBJECTIVE To establish cause-specific death rates, by age and sex in insulin-treated diabetic individuals living in Canterbury, NZ. METHODS Insulin-treated diabetic subjects on the Canterbury Diabetes Registry were followed over 20 years and vital status determined. Following notification of deaths, age- and sex-specific mortality rates, and sex-specific mortality ratios (SMRs) were calculated. RESULTS During follow-up 966 diabetic subjects contributed 13,495 person-years and 525 deaths occurred (261 females and 264 males). At all ages mortality rates were considerably higher than expected mortality. Relative mortalities were increased for cardiovascular (SMR women 3.73, 95% CI: 3.16-4.30; men 3.27, 95% CI: 2.76-3.78), renal (SMR women 5.55, 95% CI: 2.53-8.57; men 7.15, 95% CI: 3.40-10.90), respiratory disease (SMR women 3.31, 95% CI: 1.98-4.63; men 2.32, 95% CI: 1.41-3.23) and malignancy (SMR women 4.99, 95% CI: 2.99-6.99; men 2.19, 95% CI: 1.42-2.96) with cardiovascular disease accounting for the single greatest cause of excess death at all ages. CONCLUSIONS Mortality rates for diabetic individuals remain high, resulting in shortened life spans relative to the general population. To reduce these death rates attention must be paid to the early detection and treatment of CVD and associated risk factors.