To investigate differences in severity and specificity of cognitive impairment among various neurodegenerative diseases, we tested groups of patients presenting with senile dementia of the Alzheimer's type (SDAT) (n = 44), progressive supranuclear palsy (PSP) (n = 45), Huntington's disease (HD) (n = 35), and Parkinson's disease (PD) (n = 164), with an extensive neuropsychologic battery. We found dementia, as defined by a global intellectual performance 2 SD lower than mean control values, in 93% of patients with SDAT, 66% of patients with HD, 58% of patients with PSP, and 18% of patients with PD. Specific features of cognitive impairment distinguished the four groups of patients once they were matched for level of intellectual deterioration. This study shows the frequency of dementia in predominantly subcortical degenerative diseases and indicates that "subcortical dementia," rather than being a homogeneous entity, should be divided into specific subtypes of cognitive impairment related to different underlying pathology.