In an ex vivo method of differentially diagnosing dementias selected from a group of demetias, which includes vascular dementias and frontotemporal lobe degenerations, a concentration of at least one carboxy-terminally truncated amyloid .beta. peptide species as a biomarker is determined in a body fluid obtained from a patient, the at least one carboxy-terminally truncated amyloid .beta. peptide species being selected from the group of species consisting of A.beta.1-38, A.beta.1-37 and A.beta.1-39; and the concentration of the at least one carboxy-terminally truncated amyloid .beta. peptide species is compared to a threshold concentration value for the respective one carboxy-terminally truncated amyloid .beta. peptide species.