Is the incidence of dementing illness changing?

We performed a time trend study of incidence of dementing illness in Rochester, Minnesota. We ascertained age- and sex-specific incidence rates for the five quinquennia, 1960 through 1984. The incidence rates sharply increased with advancing age, reaching a high of 2,922/100,000 person years in the group 85 years and older. For dementia caused either solely or predominantly by Alzheimer's disease, this figure was 2,600/100,000 person years for the oldest age group. There were no significant differences in the incidence of dementing illness between men and women. In the oldest age groups in the last two quinquennia of study, there appears to be a trend toward increasing incidence rates. Over the years, the proportion of cases attributed to dementia due to unknown causes has decreased while the proportion of cases attributed to Alzheimer's disease has increased.