The invention is directed to methods of treatment of Alzheimer's disease and other tauopathies, via the administration of antibodies having specificity to abnormal forms of tau protein, the antibodies showing no binding and/or reactivity to a normal tau protein and being administered under conditions and in amounts effective to prevent or treat Alzheimer's disease or other tauopathies. In certain embodiments, the antibodies are selective for soluble truncated tau protein truncated at (i) its C-terminus after the glutamic acid residue Glu391, or (ii) at the aspartic acid residue Asp421, or (iii) at its N-terminus at the aspartic acid residue Asp13, or (iv) a combination of (i)-(iii). Further aspects of the invention are directed to the administration of an immunogen comprising an abnormal tau, preferably a soluble truncated tau.