The purpose of this research was to assess the frequency and severity of neuropsychiatric and behavioral symptoms and to examine the association between preexisting medical conditions and specific neuropsychiatric symptoms in demented individuals. We studied 211 demented subjects (87.7 percent male) who were participants in epidemiological studies of dementia. Using the Neuropsychiatric Inventory (NPI), we assessed the frequency and severity of neuropsychiatric symptoms. We collected medical history information during a structured telephone interview. Our analyses focused on determining prevalence of neuropsychiatric symptoms by dementia diagnosis and severity. We also examined the association of history of head injury, alcohol abuse, and stroke with development of neuropsychiatric symptoms. We found that neuropsychiatric symptoms were common, with approximately three-fourths of the subjects exhibiting at least one symptom during the preceding month. Apathy (39.3 percent), agitation (31.8 percent), and aberrant motor behavior (31.1 percent) were the most frequent symptoms. Frequency and severity of symptoms were similar for the all-dementia and Alzheimer's disease-only groups, neuropsychiatric symptoms varied by severity of dementia, but generally not in a consistent ordinal pattern. History of alcohol abuse, head injury, or stroke was associated with presence of specific neuropsychiatric symptoms in dementia. While psychiatric symptoms are common in dementia, they also vary by type and severity of dementia. The finding that certain medical conditions may increase risk for specific types of neuropsychiatric symptoms expands our knowledge of the natural history of dementia and should improve management of dementia in medically ill patients. Our results may also shed light on mechanisms that underlie neuropsychiatric symptoms.