1. The association between plaque counts in sections of cerebral cortex and measures of intellectual and personality functioning undertaken in elderly subjects during life has been studied.
2. There was no evidence that degenerative changes had contributed significantly to the causation of illness in patients with "functional" psychiatric disorders or delirious states.
3. There is a highly significant correlation between mean plaque counts and scores for dementia and performance in psychological tests. The findings suggest that psychological and pathological indices are closely related to one another, possibly through their common association with the underlying degenerative process in the brain.
4. Among severely demented subjects and those diagnosed clinically as "senile dements", correlations between psychological and pathological measures decline sharply. However, pathological differences between normal, mildly demented, and severely demented subjects appear to be of a quantitative nature. The possibility that there are qualitative differences in this group, inaccessible to present methods of examination, cannot be excluded.