Background: Longevity is increasing among persons with intellectual disability (ID) simultaneously with that of the general population. There is a need to update prevalence figures of ID among the elderly. The aim of this study was to estimate the prevalence of ID among the elderly in Finland. Method: We combined data from 8 national health and social benefits registers to calculate the ID prevalence for age cohorts separated by 1-year intervals. We corrected a discontinuity due to loss of diagnostic information in the main registers at retirement age (65 years) by comparing the age distribution to that of the entire population. Results: The distribution correction more than doubled the number of persons with ID in this age group, resulting in an apparent prevalence of 0.75%. Conclusions: The 1-year interval between age cohorts proved useful in detecting discontinuities and permitted estimation. The new estimate for the prevalence of ID among the elderly is higher than those previously published, but congruent with the increasing life expectancy of elderly persons with ID.