The study objective is to evaluate the effects of memantine treatment on communication abilities and other cognitive abilities in moderate to severe DAT patients.

Inclusion Criteria:

• Male or female outpatient at least 50 years of age and at least 8 years of education.
• The patient should have German as a mother-tongue or at least speak the language fluently in order to achieve similar scores in verbal tasks.
• Current diagnosis of probable Alzheimer's disease consistent with NINCDS- ADRDA criteria or with DSM IV TR criteria for Dementia of the Alzheimer's type.
• Mini Mental State Examination (MMSE) total score is less than 20
• Patients treated with any acetylcholinesterase inhibitor (AChEI) may be included, when treatment has started at least 6 months prior screening and was stable during the last 3 months.

Exclusion Criteria:

• Any type of evident aphasia, which may interfere with patient's communication difficulties caused by Alzheimer's disease
• History of severe drug allergy, or hypersensitivity, or patients with known hypersensitivity to amantadine, lactose.
• Evidence (including CT/MRI results) of any clinically significant central nervous system disease other than Alzheimer's disease.
• Modified Hachinski Ischemia score greater than 4 at screening.
• Current evidence of clinically significant systemic disease
• Known or suspected history of alcoholism or drug abuse within the past 10 years.
• Previous treatment with memantine or participation in an investigational study with memantine.