© 2018 Elsevier Ltd Background: Diagnosis of Alzheimer's disease in Down syndrome is challenging because of the absence of validated diagnostic biomarkers. We investigated the diagnostic performance of plasma and CSF biomarkers in this population. Methods: We did a cross-sectional study of adults aged 18 years and older with Down syndrome enrolled in a population-based health plan in Catalonia, Spain. Every person with Down syndrome assessed in the health plan was eligible to enter the Down Alzheimer Barcelona Neuroimaging Initiative, and those with a plasma or CSF sample available were included in this study. Participants underwent neurological and neuropsychological examination and blood sampling, and a subset underwent a lumbar puncture. Adults with Down syndrome were classified into asymptomatic, prodromal Alzheimer's disease, or Alzheimer's disease dementia groups by investigators masked to biomarker data. Non-trisomic controls were a convenience sample of young (23–58 years) healthy people from the Sant Pau Initiative on Neurodegeneration. Amyloid-β (Aβ) 1–40 , Aβ 1–42 , total tau (t-tau), 181-phosphorylated tau (p-tau; only in CSF), and neurofilament light protein (NfL) concentrations were measured in plasma with a single molecule array assay and in CSF with ELISA. Plasma and CSF biomarker concentrations were compared between controls and the Down syndrome clinical groups. Diagnostic performance was assessed with receiver operating characteristic curve analyses between asymptomatic participants and those with prodromal Alzheimer's disease and between asymptomatic participants and those with Alzheimer's disease dementia. Findings: Between Feb 1, 2013, and Nov 30, 2017, we collected plasma from 282 participants with Down syndrome (194 asymptomatic, 39 prodromal Alzheimer's disease, 49 Alzheimer's disease dementia) and 67 controls; CSF data were available from 94 participants (54, 18, and 22, respectively) and all 67 controls. The diagnostic performance of plasma biomarkers was poor (area under the curve [AUC] between 0·53 [95% CI 0·44–0·62] and 0·74 [0·66–0·82]) except for plasma NfL concentrations, which had an AUC of 0·88 (0·82–0·93) for the differentiation of the asymptomatic group versus the prodromal Alzheimer's disease group and 0·95 (0·92–0·98) for the asymptomatic group versus the Alzheimer's disease dementia group. In CSF, except for Aβ 1–40 concentrations (AUC 0·60, 95% CI 0·45–0·75), all biomarkers had a good performance in the asymptomatic versus prodromal Alzheimer's disease comparison: AUC 0·92 (95% CI 0·85–0·99) for Aβ 1–42 , 0·81 (0·69–0·94) for t-tau, 0·80 (0·67–0·93) for p-tau, and 0·88 (0·79–0·96) for NfL. Performance of the CSF biomarkers was optimal in the asymptomatic versus Alzheimer's disease dementia comparison (AUC ≥0·90 for all except Aβ 1–40 [0·59, 0·45–0·72]). Only NfL concentrations showed a strong correlation between plasma and CSF biomarker concentrations in participants with Down syndrome (rho=0·80; p<0·0001). Interpretation: Plasma NfL and CSF biomarkers have good diagnostic performance to detect Alzheimer's disease in adults with Down syndrome. Our findings support the utility of plasma NfL for the early detection of Alzheimer's disease in Down syndrome in clinical practice and clinical trials. Funding: Institute of Health Carlos III, Fundació La Marató de TV3, Fundació Bancaria Obra Social La Caixa, Fundació Catalana Síndrome de Down, and Fundació Víctor Grífols i Lucas.