In this cross-sectional study, analyzing CSF levels of contactin-1 and -2 in a large cohort of patients with PD and DLB, we found that the levels of synaptic protein contactin-1 in CSF were lower in PD patients compared to controls and DLB patients, but contactin-2 levels were similar across diagnostic groups. This evidence concerns the gene CNTN1 and Lewy body dementia.