The purpose of this study is to characterize the progression of disease using volumetric MRI techniques and cognitive outcome measures in patients with moderate dementia of the Alzheimer's type treated with open-label memantine.

Inclusion Criteria:

• Ambulatory male or female patients at least 50 years of age (females at least 2 years postmenopausal)
• A diagnosis of probable AD [according to the National Institute of Neurological and Communicative Disorders and Stroke and the Alzheimer's Disease and Related Disorders Associations (NINCDS-ADRDA) criteria] of moderate severity [Mini-Mental State Examination score between 15 and 20, inclusive]
• On a stable dose of Food and Drug Administration (FDA)-approved acetylcholinesterase inhibitor

Exclusion Criteria:

• Current Diagnostic and Statistical Manual of Mental Disorders-Fourth Edition (DSM-IV) Axis I disorder other than AD
• Previous imaging results not consistent with the diagnosis of AD
• Modified Hachinski Ischemia Score greater than 4
• Evidence of other neurologic disorders
• Inability to participate in MRI testing
• No clinically significant systemic disease
• A known or suspected history of alcohol or drug abuse in the past 10 years
• Taking excluded medication
• Previous treatment with commercial memantine