[Background] Presenilin-1 (PS1), the active component of the intramembrane γ-secretase complex, can be detected as soluble heteromeric aggregates in cerebrospinal fluid (CSF). The aim of this study was to examine the different soluble PS1 complexes in the lumbar CSF (CSF-PS1) of individuals with Alzheimer’s disease (AD), particularly in both symptomatic and asymptomatic genetically determined AD, in order to evaluate their potential as early biomarkers. ; [Methods] Western blotting, differential centrifugation and co-immunoprecipitation served to determine and characterize CSF-PS1 complexes. We also monitored the assembly of soluble PS1 into complexes in a cell model, and the participation of Aβ in the dynamics and robustness of the stable PS1 complexes. ; [Results] There was an age-dependent increase in CSF-PS1 levels in cognitively normal controls, the different complexes represented in similar proportions. The total levels of CSF-PS1, and in particular the proportion of the stable 100–150 kDa complexes, increased in subjects with autosomal dominant AD that carried PSEN1 mutations (eight symptomatic and six asymptomatic ADAD) and in Down syndrome individuals (ten demented and ten non-demented DS), compared with age-matched controls (n = 23), even prior to the appearance of symptoms of dementia. The proportion of stable CSF-PS1 complexes also increased in sporadic AD (n = 13) and mild-cognitive impaired subjects (n = 12), relative to age-matched controls (n = 17). Co-immunoprecipitation demonstrated the association of Aβ oligomers with soluble PS1 complexes, particularly the stable complexes. ; [Conclusions] Our data suggest that CSF-PS1 complexes may be useful as an early biomarker for AD, reflecting the pathology at asymptomatic state. ; This study was funded in part by the EU BIOMARKAPD-Joint Programming on Neurodegenerative Diseases (JPND) project, by the Instituto de Salud Carlos III (ISCIII grants PI11/03026 to JSV, PI11/02425 and PI14/01126 to JF, PI11/03035 and PI14/1561 to AL, PI08/0036 and PI12/00013 to RSV, and PI11/03023 to JLM), co-financed by the Fondo Europeo de Desarrollo Regional, under the aegis of JPND, and through CIBERNED, ISCIII. This work was also supported by the Fundació Catalana de Síndrome de Down and by a “Marató TV3” grant (20141210 to JF) and a grant from the Griffols Foundation. The funding bodies played no role in the study design, data collection and analysis, the decision to publish, or the preparation of the manuscript. We also acknowledge the support for the publication fee to the CSIC Open Access Publication Support Initiative through its Unit of Information Resources for Research (URICI). ; Peer reviewed