This phase IIIb trial is a prospective, randomized, double-blind, placebo-controlled, 36-month study comparing the length of time of progression from mild cognitive impairment (MCI) to a clinical diagnosis of Alzheimer's disease (AD) in subjects taking Exelon vs. placebo. Exelon is currently under review with the U.S. Food and Drug Administration as a treatment for Alzheimer's disease. The drug has been cleared for marketing in more than 40 countries for Alzheimer's disease to date, including all 15 member states of the European Union, New Zealand, Argentina, Brazil and Mexico.