The present study examined the prevalence of dementia in the Nagasaki Prefecture. The purposes of our investigation were (a) to study the relationship between aging and the prevalence of dementia and the ratio of Alzheimer's disease (AD) to vascular dementia (VD), (b) to understand the features of early-onset dementia as seen in patients from 60 to 65 years, and (c) to examine the recognition of dementia by family members. The subjects of the study, a total of 4,368, were all 60 years old and over and were residing in the three areas of Nagasaki Prefecture at the time of the investigation, August 1995. We adopted a two-stage design. The first-stage questionnaire that we developed was delivered to subjects, and we selected for the second stage those subjects who met the criteria outlined in the Methods section. The second-stage investigation was an interview by community nurses and psychiatrists. The prevalence of dementia in subjects 60 years and over was 6.2% (men: 5.9%; women:6.8%). The prevalence increased with age. The AD/VD ratio was 1.4, and was similar to the recent trend in Japan in that the ratio has reversed to resemble the western pattern. In regard to the family members' recognition of illness, the higher the severity of dementia, the higher the recognition ratio of family members became. Only half of these subjects were recognized as having dementia by their family members. In conclusion, the westernization of the AD/VD ratio in Japan was proved. There was little study about family recognition of dementia. In this study, it was remarkable that only half of the subjects were recognized as having dementia by their family members.