Fragile X syndrome is the most common known inherited cause of neurodevelopmental disability. Functional magnetic resonance imaging (fMRI) studies from our laboratory indicate that specific brain regions using the neurochemical, acetylcholine, show significantly reduced activation during learning. Since donepezil is a medication that enhances acetylcholine function in the brain, the purpose of this study is to determine if donepezil has any beneficial effect on behavior or cognition in subjects with fragile X syndrome.