Patients with dementia often manifest troublesome and disruptive behaviors in addition to intellectual impairments. This study evaluated behavioral disturbances in 126 demented patients examined sequentially, using questionnaires administered to primary caregivers to quantify the types and severity of behavioral disturbances. Eighty-three percent of the patients exhibited one or more of the targeted behaviors. The most common troublesome and disruptive behaviors clustered into three categories: aggressive, ideational, and vegetative. The prevalence and severity of the behaviors increased with global severity of dementia, but did not differ in either frequency or type when patients with three diagnoses were compared: Alzheimer's disease (AD), multi-infarct dementia (MID), and mixed AD and MID (MIX). The occurrence and severity of the target behaviors correlated modestly with the severity of dementia. Impairments of mental status correlated weakly with only a single troublesome and disruptive behavior--assaultiveness. These results suggest that troublesome and disruptive behaviors are a very frequent component of dementing disorders, are related to disease severity, and parallel (but are probably not determined by) intellectual deficit.