Dementing illnesses such as Alzheimer's disease (AD) progressively devastate human brain tissue and consequently the health and lives of people afflicted by these disorders. One of the greatest clinical challenges posed by dementia is establishing an approach to ensure its early identification and accurate diagnosis-thus making it possible to treat and, potentially, arrest the development of disease before a substantial amount of brain tissue has been permanently destroyed. The data generated by neuroimaging studies conducted over the past two decades show PET with [(18)F]fluorodeoxyglucose (FDG) to be exceptionally well-suited to meeting this challenge. The regional metabolic patterns imaged with FDG-PET enable sensitive diagnosis of AD, and reveal pathophysiologic alterations even before they lead to symptomatic expression. The accuracy of PET in identifying early AD, and distinguishing it from other etiologies of cognitive impairment, exceeds that of CT, MRI (qualitative or quantitative), and SPECT, as well as that of expert clinical evaluation based on history, physical examination, cognitive testing, and blood laboratory values. Recent developments in instrumentation and radiopharmaceutical distribution have made obtaining scans of cerebral metabolism achievable in routine clinical settings, including most hospitals in which Nuclear Medicine services are provided, for less than the cost of a single year of anticholinesterase therapy or a single month of lost productivity. The need and opportunity are thus present for making a fundamental change in the current approach to evaluating patients for dementia.