In this autopsy study, we show consistent results that plasma GFAP is sensitive to concomitant ADNC in LBSD with autopsy‐confirmed αSyn: antemortem plasma GFAP was significantly higher in αSyn+AD than αSyn, it was associated with higher burden of postmortem β‐amyloid (even after covarying for gliosis), and was associated with worse antemortem MMSE performance. The gene discussed is GFAP; the disease is Alzheimer disease.