The macroscopical, histological, ultrastructural and immunocytochemical features of progressive supranuclear palsy (PSP) are reviewed. Recent investigations have revealed important differences in the distribution, ultrastructure and immunocytochemical profile of neurofibrillary tangles in PSP and in Alzheimer's disease. Cortical involvement, as demonstrated by the presence of tangles and neuropil threads has extended the neuropathological spectrum of PSP. Quantitative assessments of neuronal populations show neuronal loss, not only in various nuclei of the brainstem, diencephalon and cerebellum, but also in other areas, including the nucleus basalis of Meynert, substantia nigra and neostriatum. A new classification, based on neuropathological criteria, is suggested in order to take into consideration the phenotypic heterogeneity of PSP. This new classification distinguishes three types: typical, atypical and combined cases. Typical (Type 1) cases conform to the original definition of PSP. Type 2, atypical cases are variants of the histological changes characteristic of PSP: either the severity or the distribution of abnormalities, or both of these deviate from the typical pattern. Cases with combined pathology belong to type 3 group: in these the typical pathology of PSP is accompanied by lesions characteristic of another neurodegenerative or vascular disease.