Argyrophilic and tau-positive abnormal structures occurring in glial cells are called glial fibrillary tangles. In the astrocyte, a conspicuous tau-positive structure is known to appear in progressive supranuclear palsy (PSP). In this report, another type of argyrophilic and tau-positive astrocytes is reported. The morphology of this new type is quite different from that of the previously reported tau-positive astrocyte in PSP and they are designated here as thorn-shaped astrocytes (TSA). TSA have an apparently argyrophilic cytoplasm with a few short processes and often have a small eccentric nucleus, whose appearance resembles that of a reactive astrocyte. Immunohistochemically, TSA are positive to anti-tau antibodies but are negative for ubiquitin. Simultaneous immunostaining revealed the coexistence of tau and glial fibrillary acidic protein epitopes in the same cytoplasm. Electron microscopically, bundles of 15-nm straight tubules were included in the cytoplasm together with abundant glial filaments. In the vicinity of a cluster of TSA, related structures of perivascular or subpial tau-positive linings, which correspond to astrocytic end-feet, are sometimes observed. In almost all cases, a few TSA are generally located in a confined area of subpial and subependymal regions. Although TSA appear to be intimately associated with some diseases, they are also found in a wide range of cytoskeletal disorders including the aged brain with neurofibrillary tangles. TSA are presumed to be a secondary induced product in relation to astrocytic reaction.