The Memory Impairment Study is the first such AD prevention clinical trial carried out by NIH, and will be conducted at 65-80 medical research institutions located in the United States and Canada. This study will test the usefulness of two drugs to slow or stop the conversion from MCI to AD. The trial will evaluate placebo, vitamin E, and donepezil, an investigational agent approved by the Food and Drug Administration for another use. Vitamin E (alpha-tocopherol) is thought to have antioxidant properties, and was shown in a 1997 study to delay important dementia milestones, such as patients' institutionalization or progression to severe dementia, by about seven months.