This is a multi-center, randomized, double-blind placebo-controlled safety study conducted in 2 study cohorts. In Cohort 1, subjects with Alzheimer's disease (n=250) will receive Dimebon 20 mg or placebo TID for 26 weeks. In Cohort 2 AD subjects (n=500) will be treated with Dimebon 20 mg or placebo TID for 12 weeks After completion of the randomized portion of the study, subjects in both Cohorts will have the opportunity to enroll in a Dimebon open label extension study.