Approximately 300 participants from 25-35 clinical trial centers in the United States will be enrolled. Participation will include men and women with a diagnosis of probable Alzheimer's disease, age 55 or older, weighing at least 40 kg (88.2 lbs.), residing in the community at baseline, Mini Mental State Examination (MMSE) 10-20 inclusive, who have not experienced agitation or psychosis since the onset of their illness and who do not require treatment with psychotropic medications with the exception of antidepressants used only for treatment of depressive symptoms and limited use of sedatives for sleep. Participants, their relatives, guardians or authorized representatives and informants will be given ample opportunity to inquire about details of the study. Informed consent forms covering consent for the trial itself as well as the genetic research and biological sample storage and MRI scans will be provided to protect the patient's rights and confidentiality.