To investigate the age-associated changes in physical and mental functions, epidemiological surveys were conducted for a random sample (N = 3,704) of residents aged 65 years and over in Sendai City, Japan, in 1988 and 1991. The frequency of dementia was derived from the epidemiological surveys. The prevalence of dementia aged 65 years and over in 1988 was 5.13 per 100 persons with age-specific rates of 0.78,1.89,6.02,11.95 and 26.31 per 100 persons for the age groups 65-69,70-74, 75-79, 80-84, and 85+, respectively. The incidence during 1988-1991 was 1.74 per 100 person-years with age-specific rates of 0.73, 0.87, 2.29, 3.62, and 8.48 per 100 person-years, respectively. Using the Japan population in 1985, the age-standardized rates for females were higher than those for males for both prevalence and incidence. The prevalence in this study was consistent with those of other population-based studies in urban Japan. There are few studies of the incidence of dementia in Japan, and this is the first such study based on a longitudinal design in an urban area of Japan. J Epidemiol, 1993; 3 : 83-89.
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