The prevalence of dementia in subjects 65 years and older in North America is approximately 6%-10%, with Alzheimer's disease (AD) accounting for two-thirds of these cases. If milder cases are included, the prevalence rates double. Both causative and associative genes for AD have now been identified. The search for nongenetic risk factors has been less conclusive. Only age and family history of dementia are consistently associated with AD in all studies, but putative, protective agents such as estrogen, nonsteroidal anti-inflammatory agents (NSAIDs), and vitamin E are now undergoing clinical trials.