Dementia is one of the common causes that lead to dependence of senior citizens in daily living. Clarifying the features of the elderly with dementia is instrumental in planning for their effective care and support in a community, and for attempts at prevention. Our purpose was to investigate the impact of sociodemographic factors among the elderly with the presumptive diagnosis of dementia. We carried out a survey annually from 1998 to 2002 in a dynamic cohort of community-dwelling individuals aged 65 years or older. Of the 945 subjects, 782 were eligible for study because at the first interview they were asymptomatic for dementia and not institutionalized. We found no significant difference in a 5-year average incidence rate between genders. However, the risk of developing dementia increased with age. The study population was categorized into three groups of living arrangement: those living with spouse and others, those living alone, and those living with persons other than the spouse. The incidence rate of dementia among the elderly who lived with spouse and others was significantly lower than for those among the other groups. This was also notable in the subjects without a history of stroke, even after adjustment for age and gender. This result indicates that living with spouse might have an important benefit in reducing the risk of developing dementia, although this effect would vary with the type of dementing disease. We suggest that preventive measures in clinical and community care of the elderly should focus on interactive social conditions such as living environments.