Pilot Study of Aerobic Exercise in Early Alzheimer's Disease(AD)

Inclusion Criteria: * Informed consent provided by the participant or the participant's legally acceptable representative * Age 55 years or older * Diagnosis of Probable AD according to the National Institute of Neurological and Communicative Disorders and Stroke-Alzheimer's Disease and Related Disorders Association (NINCDS-ADRDA) * Clinical Dementia Rating (CDR) of 0.5 (very mild) or 1 (mild dementia) * Mini-Mental Status Exam Score of 16 to 30, inclusive. * Rosen Modified Hachinski score of 4 or less * Community dwelling with a caregiver able and willing to accompany the participant on all visits, if necessary. The caregiver must visit with the subject more than five times a week for the duration of the study. * Underactive or sedentary as determined by a Telephone Assessment of Physical Activity. * Adequate visual and auditory abilities to perform all aspects of the cognitive and functional assessments * Stable doses of medications for at least 30 days prior to screening. Cholinesterase inhibitors and memantine are allowed provided the participant is on stable doses without clinically significant side effects for 60 days. Additionally, the participant and caregiver will agree, barring unforeseen circumstances, to continue the same regimen for the trial duration. * Likely to participate in all scheduled evaluations and complete the 26 week program. Exclusion Criteria: * CDR 0.5 uncertain dementia (i.e., not meeting NINCDS-ADRDA criteria). * Significant neurological disease, other than AD, that may affect cognition * MRI or CT scan indicative of significant abnormalities that may explain cognitive decline (e.g., multiple lacunar infarcts or a single prior infarct \> 1 cm3, micro-hemorrhages or evidence of a prior hemorrhage \> 1 cm3, evidence of cerebral contusion, encephalomalacia, aneurysm, vascular malformation, or space-occupying lesion such as an arachnoid cyst or brain tumor) * Current clinically significant major psychiatric disorder (e.g., Major Depressive Disorder) according to DSM-IV criteria or significant psychiatric symptoms (e.g., hallucinations) that could impair the completion of the study. Individuals with depressive symptoms (Geriatric Depression Scale \< 6) and who do not meet criteria for major depression will be eligible. * Current clinically-significant systemic illness that is likely to result in deterioration of the patient's condition or affect the patient's safety during the study * History of clinically-evident stroke * Clinically-significant infection within the last 30 days * Myocardial infarction or symptoms of coronary artery disease (e.g., angina) in the last two-years. * Uncontrolled hypertension within the last 6 months * History of cancer within the last 5 years (except non-metastatic basal or squamous cell carcinoma) * History of drug or alcohol abuse as defined by DSM-IV criteria within the last 2 years * Presence of contraindications for MRI scanning (pacemaker, aneurysm clips, artificial heart valves, metal fragments, foreign objects) or claustrophobia * Insulin-dependent diabetes mellitus * Significant pain or musculoskeletal disorder that would prohibit participation in an exercise program