The aim of the present study was to investigate cause-specific mortality in people with intellectual disability (ID). It was based on a 35-year follow-up study of a nation-wide population of 2369 subjects aged between 2 and 97 years. The 1095 deceased people had accumulated 64 539 person-years. The research took the form of a prospective cohort study with mortality follow-up. Observed and expected deaths were calculated as standardized mortality ratios using the Finnish general population as the reference. Cause-specific mortality ratios were calculated by the level of ID, sex and age. The three most common causes of death were cardiovascular diseases, respiratory diseases and neoplasms. Disease mortality was high up to 40 years of age, but did not increase thereafter. The difference between sexes in cause-specific mortality was smaller than in the general population. Cause-specific mortality differed significantly from the general population, with reduced mortality from neoplasms and external causes, but ageing individuals with mild ID had similar mortality patterns to the general population. The disparities in the cause-specific mortality between younger people with ID and the general population fade with advancing age, producing similar health risks. In preventative work, special attention should focus on common diseases and accidents in the community.