Decreased delayed recall, decreased amyloid-β peptides (Aβ1-42), and increased tau protein concentration in cerebrospinal fluid (CSF) are generally regarded to be valid neuropsychological and biological markers for Alzheimer's disease (AD). Previous studies failed to demonstrate clear-cut correlations between neuropsychological impairment and CSF markers. In this study we test recent models of disease progression, that propose that changes in CSF biomarkers already reach a plateau in a preclinical phase, before cognitive decline begins, that is, even before MCI can be diagnosed. We recruited 73 patients with probable AD (n=36) and mild cognitive impairment (MCI) (amnesic MCI=25; non-amnesic MCI=12). We used the CERAD-NP, a widely used neuropsychological battery with norms for different age and education groups, and additional neuropsychological tests for assessing the cognitive profile of these patient groups. We found a significant correlation between Aβ1-42 in the CSF and memory performance for amnesic MCI patients, but not for non-amnesic MCI and AD patients. All other correlations between cognitive tasks and Aβ1-42 were not significant. Tau protein concentration in the CSF was not correlated with any neuropsychological marker in any of the patients groups. We conclude that the decrease of Aβ1-42 in the CSF mirrors disease progression during the early stages up into AD and therefore is not restricted to the preclinical phase. The decrease of Aβ1-42 reaches a plateau only in the full blown demented syndrome and further functional disease progression is then related to neurodegeneration without further reduction of Aβ1-42 in the CSF.