Younger people with dementia present a unique challenge to modern Japanese society and those individuals who care for them. Although illnesses causing dementia occur much less commonly in younger than older people, it was estimated that there were about 30,000 affected younger people in 1995 in Japan. For younger people with dementia, effects on families, the presence of dependent young children, and the economic implications are particular challenges. In this article, firstly, epidemiological findings regarding illnesses causing dementia in younger people were described. Secondly, the three major degenerative dementia forms that often develop in presenescence were reviewed. Thirdly, the issue of service planning for such cases was discussed.