The differential diagnosis of the dementia syndrome may pose a difficult clinical problem, since the most common dementia, Alzheimer's disease (AD), is marked by normal laboratory tests. Neuroimaging has played an important role in evaluating the demented patient, and its uses are growing. Computed tomography (CT) is useful for excluding reversible and treatable causes of dementia, such as subdural hematoma and tumor. More recently, magnetic resonance imaging (MRI) has improved our ability to diagnose vascular disease and may show the presence of cerebral infarcts and white matter disease not visible on CT. Single photon emission computed tomography (SPECT) and positron emission tomography (PET), techniques that visualize such cerebral functions as glucose metabolism and blood flow, may provide positive evidence supportive of the diagnosis of AD.