Pin1, a peptidyl-prolyl isomerase binds to mitotic serine or threonine phosphoproteins. In Alzheimer's disease (AD) evidence points to the reactivation of mitosis in vulnerable neurons. Tangles composed of hyperphosphorylated tau contain phosphorylated Thr231 (pThr231 tau), which occurs to a greater extent in the AD brain than in the normal brain, and Pin1 has been shown to bind pThr231 tau. Here, Pin1 distribution in AD, and its colocalization with pThr231 tau in AD, FTDP-17 (P301L), Pick's disease (PiD), and PSP was investigated using TG-3, a monoclonal antibody to conformationally altered pThr231 tau. The Pin1 antibody A-20 detected granular Pin1 staining in AD brains, but not in normal brains. A-20 immunoreactive granules colocalized with TG-3-stained granules but not with TG-3-stained pretangles, tangles, or Pick bodies in AD, PiD, and FTDP-17 (P301L). Pin1 granules were sparse in PSP, and rarely did A-20 colocalize with TG-3. The appearance of Pin1 granules in the early stages of AD, PiD, and FTDP-17 (P301L) implicates Pin1 in their pathogenesis but not in PSP.