Although most dementias are due to neurodegenerative or vascular disease, it is important to diagnose immunologically mediated dementias quickly because they can be both rapidly progressive and readily treatable. They usually affect function of limbic and cortical structures, but subcortical involvement can also occur. Because of the variety of symptoms and the rapid course, these dementias present a particular challenge to the clinician and may require evaluation and intervention in the inpatient setting. Diagnostic workup typically reveals evidence of an autoimmune process and, in some cases, cancer. In contrast to the neurodegenerative processes, many of the immunologically mediated dementias respond to immunomodulatory therapy.