Our finding of increased plasma NFL in people with PD and poor outcomes, and correlated with both cognition and follow-up motor scores, is consistent with previous work showing higher CSF NFL concentrations correlated with shorter survival and worse motor symptoms in PD.94 Higher plasma NFL in people with PD and established cognitive impairment,28,29,95 correlating with rates of cognitive decline,29 has also previously been found. Here, NEFL is linked to Parkinson disease.