GDI1 and Alzheimer disease: In the total sample, lower global synaptic density was associated with higher levels of AP2B1, neurogranin, γ-synuclein, GDI-1, PEBP-1, syntaxin-1B, and syntaxin-7 but not with the levels of the neuronal pentraxins or 14-3-3 zeta/delta.<h4>Conclusion</h4>Reductions of synaptic density found in AD compared to CN participants using [<sup>11</sup>C]UCB-J PET were observed to be associated with CSF biomarker levels of synaptic proteins.<h4>Highlights</h4>A panel of synaptic proteins was quantified in the CSF using mass spectrometry.