PET Changes in Alzheimer's Disease (AD)

Inclusion Criteria: * Males and females with a minimum high school education and between the ages of 50 and 90 will be selected for Groups 1 and 2. For Group 3, normal subjects between the ages of 20 and 49 years of age will be selected. The Groups 1 and 2 will be balanced for age, and all three groups balanced for gender and ApoE genotype. * Discontinuance of all psychotropic and/or cognitively active medication at least four weeks prior to evaluation. Exclusion Criteria: * Past history or MRI evidence of brain damage including significant trauma, stroke, hydrocephalus, lacunar infarcts, seizures, mental retardation or serious neurological disorder. * Significant history of alcoholism or drug abuse. * Any history of psychiatric illness (e.g., schizophrenia, mania or depression). * Any focal signs or significant neuropathology. * A score of 4 or greater on the Modified Hachinski Ischemia Scale, indicative of cerebrovascular disease. * A total score of 16 or more on the Hamilton Depression Scale to exclude possible cases of primary depression. * Evidence of clinically relevant hypertensive, cardiac, pulmonary, vascular, metabolic or hematologic conditions. Specific exclusion will be made for individuals with fasting glucose levels \>110 mg/dl. * Physical impairment of such severity as to adversely affect the validity of psychological testing. * Hostility or refusal to cooperate. * Any prosthetic devices (e.g., pacemaker or surgical clips) that could be affected by the magnetic field employed during MRI imaging. * Evidence of cognitive or memory impairment reaching early AD levels at the initial evaluation. At baseline, delayed paragraph recall z-scores \> 2 below the reference group.