As the life expectancy of populations is extending, interest in age-associated diseases is increasing, particularly for those disorders that result in debilitating conditions and, thus, require health policy planning. Both health policy planning and the research for possible etiologies are important factors in the study of Alzheimer's disease. The contribution of epidemiologic studies to both of these aspects has been reviewed. Such investigations should be conducted only on well-defined disorders, as is discussed in this article.