Corticobasal degeneration (CBD) is a rare, progressive neurological disorder characterized by widespread neuronal and glial accumulation of abnormal tau protein. Using immunohistochemistry we analyzed tau epitope expression and phosphorylation state in CBD and compared them to cytoskeletal changes in Alzheimer's disease (AD) and progressive supranuclear palsy (PSP). Epitopes spanning the entire length of the tau protein were present in CBD inclusions. An antibody against the alternatively spliced exon 3 did not recognize cytoskeletal lesions in CBD, but did in AD and PSP. Tau epitopes from each region of the molecule were present in cytoskeletal inclusions in CBD, including gray matter astrocytic plaques, gray and white matter threads, and oligodendroglial inclusions. As in AD, tau from CBD was highly phosphorylated. Antibodies that recognized phosphorylated tau epitopes reacted with material from CBD in a highly phosphatase-dependent manner. Again, all types of inclusions contained phosphorylated epitopes. We conclude that abnormal tau protein in CBD comprises the entire tau molecule and is highly phosphorylated, but is distinguished from AD and PSP by the paucity of epitopes contained in the alternatively spliced exon 3.