Amyloid β protein 40 (Aβ40) and 42 (Aβ42), major components of senile plaque amyloids, are physiological peptides present in the brain, cerebrospinal fluid (CSF) and plasma. The levels of CSF Aβ40 and Aβ42(43) show a U-shaped natural course in normal aging. The increase of Aβ42(43) over 60 years of age is inhibited in Alzheimer's disease (AD). This specific alteration of CSF Aβ42(43) correlates with Aβ deposits in the AD brain providing a biological basis for a biomarker of AD. In the GTT2 study, assays of the CSF Aβ ratio [(Aβ40/ Aβ42(43)] showed a diagnostic sensitivity (59%) specificity (88%) The levels of the Aβ ratio increased from early to late stages of AD. Combination assays of CSF tau and Aβ ratio provided further efficient diagnostic sensitivity (81%) reliability of the assay may prompt worldwide usage of these CSF biomarkers for Alzheimer's patients.