The purpose of this study is to evaluate the potential benefits of CERE-110 in the treatment of Alzheimer's disease. CERE-110 is an experimental drug that is designed to help nerve cells in the brain function better. CERE-110 uses a virus to transfer a gene that makes Nerve Growth Factor (NGF), a protein that may make nerve cells in the brain healthier and protect them from dying. The virus used in CERE-110 does not cause disease in people. CERE-110 has been carefully studied in laboratory animals and is in the early stages of being tested in people. Fifty patients with mild to moderate Alzheimer's disease will participate in this study. Half of the study subjects will have CERE-110 injected into the brain during a surgical procedure, while the other half will undergo a "placebo" surgery where no medication will be injected. All study participants will be followed for at least two years after surgery.