The increasing number of dementia is of major public health concern. This study aims to calculate the projected number and prevalence of dementia in Japan, using data from the Toyama Dementia Survey.

The Toyama Dementia Survey was conducted 6 times in 1983, 1985, 1990, 1996, 2001, and 2014. In the 2014 survey, the subjects were randomly chosen from residents aged 65 or more in Toyama prefecture, with a sampling rate of 0.5%. Of those, 1303 men and women agreed to participate (participation rate: 84.8%). An interview with a screening questionnaire was conducted by public health nurses. Psychiatrists and public health nurses further investigated for the suspected cases of dementia and diagnosed whether the cases had dementia. The 1985–2001 surveys were conducted in a similar way, and, therefore, data from the 1985–2014 surveys were used in the analysis.

The prevalence of dementia in Toyama prefecture increased from 4.7% in 1985 to 15.7% in 2014. Using the age and sex-specific prevalence of dementia in the 2014 survey, the projected number of dementia is approximately 4.8 million (prevalence rate: 14.1%) in 2015, 6.1 million (16.7%) in 2025, and 7.2 million (19.2%) in 2035. Using the age and sex-specific prevalence of dementia as estimated by linear regression models, the projected number of dementia is approximately 4.7 million (13.9%) in 2015, 7.1 million (19.5%) in 2025, and 9.7 million (25.8%) in 2035.

The number of dementia in Japan could double in the next 20 years, which corresponds to 1 in 4 elderly people.

The authors have not supplied their declaration of competing interest.