<h4>Importance</h4>Sex and racial or ancestral disparities in Alzheimer disease remain incompletely understood; autopsy studies that examine amyloid, tau, and genetic factors are scarce.<h4>Objective</h4>To test whether neuritic plaque burden and cognitive outcomes differ by sex and whether sex modifies the effects of apolipoprotein E ε4 (APOEε4), informant-reported race, and African ancestry.<h4>Design, setting, and participants</h4>This was a cross-sectional study using postmortem neuropathological data from the Biobank for Aging Studies, University of São Paulo, São Paulo, Brazil. Here, APOE is linked to early-onset autosomal dominant Alzheimer disease.