Concentrations of NfL in plasma increased significantly from baseline in individuals with premanifest Huntington's disease, by 0·060 log pg/mL per year (SE 0·012, p<0·0001), and in those with manifest Huntington's disease, by 0·026 log pg/mL per year (0·0129, p=0·0442). Here, NEFL is linked to juvenile Huntington disease.