With the development of new treatments and the concept of mild cognitive impairment as an initial phase of Alzheimer's disease, there is an increasing demand for early and accurate diagnosis. Biological markers that make it possible to establish the diagnosis at an early stage of the disease serve this purpose. A marked increase in the concentrations of tau protein and a decrease in the concentrations of the 42-amino acid isoform of b-amyloid protein in the cerebrospinal fluid of patients with Alzheimer's disease has been observed.