Compared to PD patients with normal cognition, patients with PDD had similar plasma Aβ42 levels (median 7.89 [IQR 6.22–10.29] vs. 8.13 [IQR 6.19–10.57] pg/mL, p = 0.896), but higher levels of total tau (2.70 [IQR 1.94–3.31] vs. 1.61 [IQR 0.99–2.24] pg/mL, p < 0.001), Aβ40 (129.23 [IQR 103.24–147.70] vs. 95.37 [IQR 75.91–119.43] pg/mL, p < 0.001), and α-synuclein (16,247 [IQR 10,802–22,546] vs. 12,385 [IQR 9244–16852] pg/mL, p = 0.048). Here, MAPT is linked to Parkinson disease.