Two suggested clinical trial designs for assessing progression of Alzheimer disease are the randomized withdrawal design and the randomized start design. The most promising of these, the randomized start design, has the potential to demonstrate a delay in progression, but there remain problematic design, ethical, and statistical issues to be solved before the protocol can be used in a clinical trial. The development of biological markers of the disease process using neuroimaging or other measures also may provide a robust method of measuring disease progression and demonstrating the biological effect of a drug on the disease process.