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STUDY OBJECTIVE--Analyses of causes of mortality in people with diabetes using data form death certificates mentioning diabetes provide unreliable estimates of mortality. Under-recording of diabetes as a cause on death certificates has been widely reported, ranging from 15-60%. Using a population based register on people with diabetes and linking data from another source is a viable alternative. Data from the Office of Population Censuses and Surveys (OPCS) are the most acceptable mortality data available for such an exercise, as direct comparison with other published mortality rates is then possible. DESIGN--A locally maintained population-based mortality register and all insulin-treated diabetes mellitus cases notified to the Leicestershire diabetes register (n = 4680) were linked using record linkage software developed in-house (Lynx). This software has been extensively used in a maintenance and update cycle designed to maximise accuracy and minimise duplication and false registration on the diabetes register. Deaths identified were initially coded locally to the International Classification of Diseases, 9th revision (ICD9), and later a linkage was performed to use official OPCS coding. Mortality data identified by the linkage was indirectly standardised using population data for Leicestershire for 1991. Standardised mortality ratios (SMR) were estimated, with 95% confidence intervals. Insulin dependent diabetes (IDDM) was defined as diabetes diagnosed before age 30 years with insulin therapy begun within one year of diagnosis. All other types were considered non-insulin dependent diabetes (NIDDM). Analyses were performed for the whole sample and then for the NIDDM subgroup. Results from these analyses were similar and therefore only whole group analyses are presented. MAIN RESULTS--A total of 370 deaths were identified for the period of 1990-92 inclusive - 56% were in men and 44% in women, median age (range) 71 years (12-94). Approximately 90% of deaths were subjects with NIDDM. Diabetes was mentioned on 215 (58%) death certificates. The all causes SMRs were significantly raised for men and women for all ages less than 75 years. Ischaemic heart disease (ICD9) rubrics 410-414) accounted for 146 (40%) deaths - 41% of male and 38% of female deaths. Male and female SMRs were significantly raised for the age groups 45-64, 65-74, and 75-84 years. Cerebrovascular disease (ICD9 rubrics 430-438) accounted for 39 (10%) deaths and the SMR for women the external causes of death (ICD9 rubrics E800-E999) were also significantly raised overall and in age groups 15-44 and 45-64 years. This was not true for men, although numbers of deaths in this category were small for both men (4) and women (9). CONCLUSION--Record linkage has been used successfully to link two local, population based registers. This has enabled an analysis of mortality in people with diabetes to be performed which overcomes the problems associated with using as a sample, death certificates where diabetes is mentioned. The mortality rates and SMRs estimated should more accurately reflect the true rates than would be possible using other methods. The persisting excess mortality identified for people with diabetes is of a similar magnitude and attributable to similar causes as has been reported elsewhere in population based studies.