We extend the reasoning commonly applied to the understanding of chronic diseases and their prevention to Alzheimer's disease and other dementing disorders. The highly visible progress in finding genetic bases of Alzheimer's disease may leave a large proportion of cases unexplained. Rather, both genetic susceptibility and environmental factors are demonstrably important for disease expression. Thus, the research and health policy implication would be to target resources toward identifying and modifying environmental risk factors that might delay or prevent dementia.