This study will evaluate the safety and potential benefit of Dimebon as compared to placebo in patients with mild to moderate Alzheimer's Disease.

Inclusion Criteria:

1. Males or females greater than or equal to 50 years of age.
2. Diagnosis of Alzheimer's Disease according to Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition, Text Revision (DSM-IV TR) and Mini Mental State Exam between 10 and 24, inclusive and Modified Hachinski Ischemia Score less than or equal to 4.
3. Brain imaging such as MRI and/or CT within one year of enrollment.
4. Subjects must have a guardian or caregiver who assists the subject at least 5 days per week (at least 3 hours/day).
5. Subjects must be able to cooperate with drug administration (including the ability to ingest oral medications) and study procedures and abide by study restrictions.
6. Subjects must have at least 6 years of prior education and should have previously (in pre-Alzheimer's condition) been capable of reading, writing and communicating effectively with others.
7. Subjects must be willing and able to give informed consent or have a mentally competent legal representative authorized to provide informed consent on their behalf.
8. Residence in an assisted care facility is allowed if subject is living independently.

Exclusion Criteria:

1. Major structural brain disease
2. Major medical illness or unstable medical condition within 6 months of screening that may interfere with the subject's ability to comply with study procedures and abide by study restrictions.
3. Residence in a nursing home or assisted care facility that provides the subject with 24-hour care and supervision.
4. Women who are pregnant, nursing, or if of child-bearing potential not using a medically accepted, highly effective method of birth control.
5. Active alcohol dependence or drug abuse.
6. Use of the following medications within 60 days prior to enrollment:cognition enhancing agents, narcotic analgesics, low potency neuroleptics, antihypertensive agents with frequent Central Nervous System (CNS) side effects, anti-Parkinsonian medications, medications with central anticholinergic activity, medications for epilepsy, lithium.
7. Participation in an investigational drug or device study within 30 days prior to study entry, or 60 days prior to study entry if the investigational drug study involved therapy for Alzheimer's Disease.