In this investigation we assessed both the diagnostic accuracy and the performance in predicting cognitive decline of the combination of CSF AD biomarkers (Aβ42, t-tau, p-tau, and Aβ42/t-tau ratio) and α-syn species (t-α-syn, o-α-syn, and o/t-α-syn ratio) in a cohort of PD patients and neurological controls followed up for 2–6 years (median follow-up duration: 3 years). Here, MAPT is linked to Alzheimer disease.