Word intrusions, a behavioral abnormality previously observed in experimental subjects receiving anticholinergic medication as well as in patients with Alzheimer dementia (Alz), were studied as a diagnostic indicator of Alz in 29 elderly nursing home residents who later came to autopsy. Those who intruded words from one portion of a mental status test to another tended to have low levels of choline acetyltransferase in the cerebral cortex (median, 104.33,p less than 0.05) and large numbers of cortical senile plaques (median, 16.0,p less than 0.01). Immediate perseverations and "guesses" on a memory test were not counted. In another group of 38 patients referred for clinical neurological examination for dementia, intrusions were associated with Alz in 84% of those who exhibited them and identified 90% of patients with this presumptive diagnosis. Word intrusion appears sufficiently characteristic of Alz to be helpful diagnostically. The association of this behavioral phenomenon with low choline acetyltransferase levels and large numbers of senile plaques suggests that these changes may be important in producing the characteristic behavioral deterioration of Alz.