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. The gene discussed is GFAP; the disease is frontotemporal dementia.