To investigate potential heterogeneity in progressive supranuclear palsy (PSP), we examined 13 patients with neuropathologically diagnosed PSP. The clinical diagnosis of PSP was made in eight of these individuals, whereas probable AD was the primary diagnosis in the remaining five. In addition to PSP neuropathology, seven of the 13 patients (54%) showed concomitant pathologic changes of Alzheimer's disease (AD), Parkinson's disease (PD), or both disorders. These observations indicate that AD and PD changes coexist with PSP neuropathology in a substantive proportion of patients and provide further evidence of clinical and neuropathologic heterogeneity in neurodegenerative disorders. Moreover, our results suggest that PSP may be underdiagnosed and deserves more prominence in the differential diagnosis of dementing illness.