The exact prevalence of dementia in Denmark is unknown. Based on epidemiological data we have calculated possible estimates for the present Danish prevalence of dementia and prevalence projections through to 2030. The discrepancy between estimates based on epidemiological studies and the number of dementia diagnoses in registers indicates that dementia may be underdiagnosed. Even though results from recent epidemiological studies point to a possible decline in incidence we expect the future prevalence of dementia to increase due to an expected increase of the elderly population.