The incidences of senile dementia of Alzheimer type (SDAT) and multi-infarction dementia (MID) were studied in a total Swedish population, the Lundby project. The study is prospective and covers a 25-year period. The incidence rates per year of contracting SDAT or MID and the probability in each 10-year age interval of contracting dementia in the elderly were calculated, as well as the cumulative risk up to a certain age. The lifetime risk of contracting SDAT was for men 25.5% and for women 31.9%. The corresponding figures for MID were 29.8 and 25.1%.