An assay for the diagnosis of multi-infarct dementia and dementia of the Alzheimer type in an individual is provided. It comprises the steps of obtaining a sample from said individual being a blood sample, a platelet-containing fragment thereof, or a fragment containing platelet associated antibodies (PAA) detached from the platelets, and then determining the level of a platelet associated antibody against a 75 kD platelet-protein in said sample. A level higher than that of a control sample indicates that said individual has a high likelihood of having multi-infarct dementia or dementia of the Alzheimer type.