This is a randomized, placebo-controlled, double blind, multicenter 26-month trial of valproate therapy at a target dose of 10-12 mg/kg/day in 300 outpatients with mild to moderate Alzheimer's Disease (AD) who lack agitation and psychosis at baseline and since onset of illness. Participants will have regular clinic visits as well as telephone contacts for assessment of behavior, cognition, function, safety and tolerability. The chief secondary aim is to determine whether valproate administration to participants with AD will attenuate clinical progression of illness measured by a reduced rate of cognitive or functional decline. In addition, issues related to safety and tolerability with low-dose (10-12 mg/kg/day) therapy will be addressed. Biological specimens will be obtained to study markers selected for their relevance to the disease as well as the postulated mechanism of action of the valproate therapy. Magnetic resonance imaging (MRI) scans will be performed prior to experimental treatment and after one year in a subset of participants in order to address possible drug-placebo differences in brain volume measures.