A Donanemab (LY3002813) Study in Participants With Preclinical Alzheimer's Disease (TRAILBLAZER-ALZ 3)

Inclusion Criteria: * A Telephone Interview for Cognitive Status - modified (TICS-M) score reflective of intact cognitive functioning. * Has a phosphorylated tau (P-tau) result consistent with the presence of amyloid pathology. * Has a reliable study partner and backup study partner familiar with overall function and behavior, such as day-to-day activities and cognitive abilities. * Have adequate literacy, vision, and hearing for neuropsychological testing at screening. * Contraceptive use by men or women should be consistent with local regulations regarding the methods of contraception for those participating in clinical studies * Female participants include those who are infertile due to surgical sterilization (hysterectomy, bilateral oophorectomy, or tubal ligation), congenital anomaly such as Mullerian agenesis; or post menopausal (women 55 or older not on hormone therapy and had at least 12 months of spontaneous amenorrhea; or with a diagnosis of menopause prior to starting hormone replacement therapy. Treatment Extension: (Study Period II Placebo Group Only) * Are actively participating in Study AACM at the conclusion of Study Period III. Exclusion Criteria: * Mild cognitive impairment or dementia, or significant other neurodegenerative disease that can affect cognition. * Current serious or unstable illnesses including cardiovascular, hepatic, renal, gastroenterologic, respiratory, endocrinologic, neurologic, psychiatric, immunologic, or hematologic disease that could interfere with the analysis of the study or a life expectancy of approximately ≤5 years. * History of cancer with high risk of recurrence and preventing completion of the trial. * History of clinically significant multiple or severe drug allergies, or severe posttreatment hypersensitivity reactions (including but not limited to erythema multiforme major, linear immunoglobulin A dermatosis, toxic epidermal necrolysis, and/or exfoliative dermatitis). * Have any clinically important abnormality at screening on magnetic resonance imaging (MRI) or clinical laboratory test results that could be detrimental to the participant or study integrity. * Have any contraindications for MRI, including claustrophobia or the presence of contraindicated metal (ferromagnetic) implants/cardiac pacemaker. * Have a centrally read MRI demonstrating presence of amyloid-related imaging abnormalities (ARIA-E), \>4 cerebral microhemorrhages, more than 1 area of superficial siderosis, any macrohemorrhage or severe white matter disease at screening. * Have had prior treatment with a passive anti-amyloid immunotherapy \<5 half-lives prior to randomization. * Have received active immunization against amyloid beta (Aβ) in any other study. * Have received active immunization against Aβ in any other study. * Current or previous use of prescription medications used as treatment for mild cognitive impairment (MCI) or AD. Addendum 7 Exclusion Criteria: * Same as the main study except contraindications for florbetapir F 18 PET are exclusionary.