Findings indicate a likely greater prevalence of preclinical and prodromal Alzheimer disease (AD) today than previously anticipated. Updated estimates may inform health care planning and recruitment strategies for clinical trials of AD therapies.

• This cross-sectional study of 19 097 individuals was conducted to determine the prevalence of amyloid abnormality evaluated in cerebrospinal fluid (CSF) or on positron emission tomography (PET) scans across the clinical AD spectrum.

• Amyloid abnormality prevalence using cohort-provided cutoffs were similar to 2015 estimates for people without dementia and were found to be similar across PET- and CSF-based estimates (24%) in those with normal cognition, 27% in those with subjective cognitive decline, and 51% in those with mild cognitive impairment.

• In individuals without dementia, the CSF–based amyloid abnormality prevalence estimate that used data-driven cutoffs was 10% higher compared with the PET–based prevalence estimate that used cohort-provided cutoffs.