Misidentification delusions (MDs) are considered relatively rare psychopathologic phenomena that may occur within the context of psychiatric or neurological conditions. The purpose of this study was to assess the prevalence of MD in different types of dementia, correlate the presence of MD with demographic and clinical variables, and validate a specific questionnaire. We examined 146 subjects with Alzheimer disease, 21 with Lewy body dementia, 6 with frontotemporal dementia, and 13 with vascular dementia (subcortical type), who were consecutively enrolled in the study from 2 Memory Clinics. Patients had a mean age of 78.7±6.4 years and an Mini-Mental State Examination average score of 16.9±6.1. The Neuropsychiatric Inventory delusion subscale and a new Misidentification Delusion Questionnaire aimed at specific assessment of 11 delusional misidentification syndromes were administrated to the caregivers. On the basis of the Neuropsychiatric Inventory, MDs were present in 33.3% of the subjects, whereas according to the Misidentification Delusion Questionnaire they were present in 36.0% of the subjects. Specifically, 34.2% of Alzheimer disease, 52.4% of Lewy body dementia, and 46.1% of vascular dementia patients experienced at least 1 MD. None of the patients with frontotemporal dementia developed MD. The most frequent MD was house misidentification, followed by splitting of people and reduplicative paramnesia. Our self-administered questionnaire proved to be an accurate and specific tool for the detection of MD.