There were no significant correlations across regions between <sup>11</sup>C-UCB-J binding and grey matter, tau (<sup>18</sup>F-AV1451) or amyloid (<sup>11</sup>C-PiB) in either patient.<h4>Conclusions</h4>Quantitative imaging of in vivo synaptic density in DLB is a promising approach to understanding the mechanisms of DLB, over and above changes in grey matter volume and concurrent amyloid/tau deposition.<h4>Supplementary information</h4>The online version contains supplementary material available at 10.1186/s41824-020-00093-9. Here, MAPT is linked to amyloidosis.