This is a multicenter, double-blind, placebo controlled, randomized, outpatient, multiple dose study in male and female patients aged 50 to 89 years with mild to moderate AD. Approximately 200 study sites in the US and Canada will be involved. Patients will be randomized to receive either bapineuzumab or placebo. Each patient's participation will last approximately 1.5 years.

Bapineuzumab is a humanized monoclonal antibody, which binds to and clears beta amyloid peptide, and is designed to provide antibodies to beta amyloid directly to the patient.

Inclusion Criteria:

• Diagnosis of probable AD
• Age from 50 to less than 89
• Mini-Mental Status Exam score of 16-26 inclusive
• Brain magnetic resonance imaging (MRI) scan consistent with the diagnosis of AD
• Stable doses of medications (cholinesterase inhibitors and memantine allowed)
• Caregiver able to attend all clinic visits with patient

Exclusion Criteria:

• Significant neurological disease other than AD
• Major psychiatric disorder
• Significant systemic illness
• History of stroke or seizure, autoimmune disease, myocardial infarction within the last 2 years
• Smoking greater than 20 cigarettes per day
• Anticonvulsants, anti-Parkinson's, anticoagulant, or narcotic medications
• Prior treatment experimental immunotherapeutics or vaccines for AD
• Women of childbearing potential
• Presence of pacemakers, CSF shunts, or foreign metal objects in the eyes, skin or body