Introduction
The aim of this study was to investigate the registry-based national time trends in incidence and prevalence rates of dementia from 1996 to 2015.

Methods
We assessed annual incidence and prevalence using longitudinal data from nationwide registries on dementia status and demographics on all residents ≥ 65 years old in Denmark.

Results
Our population comprised 2 million people, of whom 152,761 were diagnosed with dementia. The age- and sex-adjusted incidence rate increased, on average, by 9% annually from 1996 to 2003, followed by a 2% annual decline, while total prevalence increased during the whole period.

Discussion
This is the first study to report continuous time trends of incidence and prevalence in an entire national population. The incidence rate has declined steadily since 2003, while the total prevalence is still increasing. Future health care planning on prevention and treatment of dementia should take these findings into account.