A population-based prevalence sample of 355 residents of Cache County, Utah, who were diagnosed with dementia, was rated on the Neuropsychiatric Inventory (NPI). Of the 355 residents, 119 had no neuropsychiatric symptoms at baseline and were, consequently, at risk for incident mental and behavioral disturbances. The NPI was readministered approximately 18 months later to 61 surviving participants. Sixty-nine percent developed at least one mental or behavioral symptom. Delusions were most common (28%), followed by apathy (21%), and aberrant motor behavior (21%). When this incidence rate of 69% was combined with a previously estimated prevalence rate of 61%, the cumulative 18-month prevalence approached 90%. These results argue for a routine assessment of psychiatric disturbances in all patients with dementia, even among those who have never experienced symptoms of mental or behavioral disturbances.