Study subjects will satisfy NINCDS-ADRDA criteria for probable AD. Dementia severity will be in the mild to moderate range, as evidenced by a Mini-Mental State Examination total score of 12-25 which is roughly equivalent to Alzheimer's Disease Assessment Scale Cognitive (ADAS-Cog) scores of 30 to 11, will include deficits in at least 2 areas of cognition, will have been present at least 1 year, and will have progressed gradually since onset. Patients must be between the ages of 50-90 with onset of dementia after age 40. The modified Hachinski Ischemia Score must be less than 4 and the brain MRI within 2 years of enrollment must be consistent with the diagnosis of AD.