An increasing number of recently described tau mutations show considerable clinical heterogeneity. The assessment of this phenotypic variation is of vital importance in the differential diagnosis of neurodegenerative diseases.To assess the neuropathologic heterogeneity in a comprehensive study of 12 brains with a tau mutation at exon 10(+16) (C-to-T) splice site from 9 families.A comprehensive neuropathologic examination has been carried out, using a wide range of tau antibodies.All brains showed frontotemporal atrophy of varying severity and pallor of the pigmented nuclei of the brainstem. The histologic changes were more extensive to include other cortical areas, the deep gray matter, and the white matter. The hallmark histologic lesions were the tau-positive neuronal and glial inclusions. In neurons, these ranged from typical neurofibrillary tangles through well-circumscribed inclusions to diffuse cytoplasmic staining. This tau pathology was complemented by the presence of large, abnormal achromatic neurons, neuronal loss, astrocytosis, and superficial status spongiosus.The distribution, type, and severity of these histologic abnormalities varied not only from case to case but also within the same brain. These brains with a common tau mutation raise important differential diagnostic problems: cases in the past might have been misdiagnosed as corticobasal degeneration or even atypical Pick disease, disorders with similar, if not identical, phenotypic manifestations.