A Study to Evaluate the Efficacy and Safety of ABBV-8E12 in Participants With Early Alzheimer's Disease

* Subject who meets the National Institute on Aging and the Alzheimer's Association (NIA-AA) clinical criteria for mild cognitive impairment or probable AD, and have: * Clinical Dementia Rating (CDR)-Global Score of 0.5 * Mini-Mental State Examination (MMSE) score of 22 to 30, inclusive * Repeatable Battery for the Assessment of Neuropsychological Status-Delayed Memory Index (RBANS - DMI) score of 85 or lower * Subject has a positive amyloid positron emission tomography (PET) scan. * Subject has a Modified Hachinski Ischemic Scale (MHIS) score of ≤ 4. * The subject has an identified, reliable, study partner (e.g., family member). * If using medications to treat symptoms related to AD, doses must be stable for at least 12 weeks prior to randomization.

* Subject has any contraindications or inability to tolerate brain magnetic resonance imaging (MRI), PET scans or lumbar puncture. * Subject has evidence of any other clinically significant neurological disorder other than early AD. * In the opinion of the investigator, the subject has any clinically significant or uncontrolled medical or psychiatric illness, or has had an infection requiring medical intervention in the past 30 days. * Subject has had a myocardial infarction, unstable angina, stroke, transient ischemic attack or required intervention for any of these conditions within 6 months of Screening.