GFAP and NfL level increases were associated with MMSE score decline (estimate [SE], -0.005 [0.002], P = .007 and -0.010 [0.003], P = .001, respectively), while p-tau217 level increases were associated with CDR-SB score worsening (estimate [SE], 0.072 [0.024], P = .003) in the EOAD group.<h4>Conclusions and relevance</h4>In this cohort study of patients with EOAD and FTD, baseline p-tau217, GFAP, and NfL were consistently associated with clinical outcomes in the EOAD group, whereas GFAP and NfL were associated with cognition only in the FTD group. This evidence concerns the gene NEFL and frontotemporal dementia.