Additional Study Information: You have been selected as a possible participant in this study because you have memory or thinking problems (cognitive impairment) or mild Alzheimer's disease. Alzheimer's disease is a slow progressive disease of the brain that affects memory and brain function. Currently available medications for Alzheimer's disease provide limited symptomatic benefit and do not significantly change the course of the illness. This study may provide further understanding of the disease and potential benefits of treatment with crenezumab for patients with Alzheimer's disease. If it works, crenezumab may slow down how fast your disease progresses. It is not expected to improve your memory over current levels. You may not get any benefit from being in the study.