We found that synaptic loss was most severe in temporal, cingulate and insular cortices affected at Braak α-synuclein stage 5 in PD, and additionally in parahippocampal regions affected at Braak α-synuclein stage 4 in PDD and DLB, and that synaptic loss associated with NfL immunoreactivity and LB density. Here, SNCA is linked to Lewy body dementia.