This invention provides methods useful in aiding in the diagnosis of Alzheimer's disease. The methods involve measuring the amount of amyloid-.beta. peptide (x-.gtoreq.41) in the cerebrospinal fluid of a patient. High levels of the peptide generally are inconsistent with a diagnosis of Alzheimer's. Low levels of the peptide are consistent with the disease and, with other tests, can provide a positive diagnosis. Other methods involve measuring the amounts of both A.beta.(x-.gtoreq.41) and tau. Low levels of A.beta.(x-.gtoreq.41) and high levels of tau are a positive indicator of Alzheimer's disease, while high levels of A.beta.(x-.gtoreq.41) and low levels of tau are a negative indication of Alzheimer's disease.