A follow-up clinical and neuropathological study of eleven mentally subnormal subjects diagnosed in life as suffering from either senile, cerebral arteriosclerotic or pre-senile dementia is reported. Of the original cohort seven have died and neuropathological examination has confirmed the diagnosis of cerebral arterisclerotic dementia in three, senile dementia in one and senile dementia of unusually early onset in another mongol patient. Autopsy was refused in one patient and in the other patient neuropathological examination revealed a diffuse sclerosis of Pelizaeus-Merzbacher type. In the four survivors the clinical diagnosis of dementia would appear to be probably correct in one patient, partially correct in another, and wrong in two patients. The study has confirmed that dementia can be diagnosed in mental defectives with a reasonable degree of accuracy and draws attention to the potential interest of neuropathological examinations in decreased psychiatrically disordered mentally subnormal subjects.