Inclusion Criteria:

• Subject must be able to understand the nature of the study and has the opportunity to have any questions answered. The subject has voluntarily signed the institutional review board (IRB)/independent ethics committee (IEC) approved informed consent, prior to the conduct of any extension study-specific procedures (including any changes occurring in the subject's current therapeutic regimen). In the absence of subject's ability to provide the informed consent, the informed consent must be obtained by a person who has the legal right to act on behalf of the subject following local regulations.
• Subject completed the 96-week treatment period of Study M15-566.
• In the investigator's opinion, subject was compliant during participation in Study M15-566.
• Subject has an identified, reliable study partner (e.g., caregiver, family member, social worker, or friend; preferably the same person for the duration of the study) who has frequent contact with the subject (at least 10 hours per week) and who will provide information as to the subject's cognitive and functional abilities. The study partner has voluntarily signed the IRB/IEC approved study partner informed consent, prior to the conduct of any extension  study-specific procedures.
• If female, subject must be postmenopausal defined as:
  • Age ≥ 55 years old with no menses for 12 or more months without an alternative medical cause; OR
  • Permanently surgically sterile (bilateral oophorectomy, bilateral salpingectomy or hysterectomy).
• If the male subject is sexually active with female partner(s) of childbearing potential, he must agree, from Study Day 1 through 20 weeks after the last dose of study drug to practice the protocol specified contraception and must refrain from sperm donation.
• All subjects with early AD who complete Study M15-566 (NCT02880956), meet all inclusion criteria, and do not meet any exclusion criteria are eligible for enrollment.
• Subject was compliant during participation in Study M15-566 (NCT02880956).
• Subject has an identified, reliable study partner who has frequent contact with the subject and who will provide information as to the subject's cognitive and functional abilities.

Exclusion Criteria: 

• The subject has any significant change in his/her medical condition since participation in Study M15-566 that could interfere with the subject's participation in Study M15-570, could place the subject at increased risk, or could confound interpretation of study results. The investigator must re-evaluate the subject for continuing participation and consider relevant factors including:
  • interim development of any clinically significant neurological, hematological, autoimmune, endocrine, cardiovascular, neoplastic, renal, hepatic, metabolic, psychiatric, pulmonary, gastrointestinal, or other major disorder;
  • interim development of contraindication to or inability to tolerate brain MRI or PET scans.
• More than 8 weeks have elapsed since the subject received his/her last dose of study drug in Study M15-566 (i.e., Week 92 visit in Study M15-566). In certain cases, subject may be eligible to enroll after approval by the TA MD.
• Subject is concurrently enrolled in another interventional clinical study (with the exception of Study M15-566) involving a therapeutic agent.
• Subject is considered by the investigator, for any reason, to be an unsuitable candidate to receive ABBV-8E12 or the subject is considered by the investigator to be unable or unlikely to comply with the dosing schedule or study evaluations.