Impaired deficit awareness is important to detect because of its clinical implications; for example, patients with impaired awareness are more likely to engage in potentially dangerous activities, such as operating the stove. This study evaluated the reliability and validity of the Dementia Deficits Scale (DDS), a new multidimensional instrument for the assessment of self-awareness of cognitive, emotional, and functional deficits associated with dementia. Three parallel forms of the DDS were independently completed by the patient, clinician, and informant. The DDS yields two measures of deficit awareness: the discrepancy between clinician and patient assessments of the patient's deficits, and the discrepancy between informant and patient assessments. The DDS demonstrated adequate reliability and good divergent and convergent validity. Clinician and informant reports each added unique information, highlighting the importance of including both perspectives in the awareness assessment process. The DDS is recommended as a reliable and valid tool for the clinical and empirical assessment of deficit awareness in patients with dementia.