Background: Determination of Alzheimer's disease (AD) by cerebrospinal fluid (CSF) biomarkers - 42-amino-acid amyloid-β (Aβ₄₂), total tau and phosphorylated tau (p-tau) - has demonstrated high validity for detecting AD neuropathological changes. However, their prognostic utility to predict the onset of dementia in predementia subjects is still questioned. We aimed to study the prospective clinical evolution of a group of subjects with subjective cognitive decline (SCD) or mild cognitive impairment (MCI) and to determine the prognostic capacity of AD CSF biomarkers. Methods: 149 subjects with MCI or SCD, not meeting dementia criteria, underwent a prospective clinical, neuropsychological and CSF biomarker study. Patients were initially classified as SCD or MCI following internationally accepted criteria. CSF sampling was obtained and analysed following consensus protocols. Neuropsychological and clinical evaluations were conducted at the follow-up. Statistical analysis considering the final clinical diagnosis, regression analysis to define risk factors and survival curves for progression were made. Results: 72.4% of subjects (83% MCI and 27% SCD) with a pathological CSF ratio (Aβ₄₂/p-tau) met criteria for dementia during the 5-year follow-up versus 18.7% of subjects from the group with a normal ratio. The pathological CSF ratio was a powerful marker of risk for AD dementia (OR 27.1; 95% CI 10.3-71.2). Kaplan-Meier survival curves showed that only 15% of subjects with a pathological CSF ratio remained free of AD dementia at 5 years of follow-up. All subjects who reverted to normal cognition presented a normal CSF profile at baseline. Conclusion: An abnormal AD CSF biomarker profile in predementia subjects is a powerful predictor of cognitive and/or functional decline in the medium term.