Undoubtedly, the future will see experimental therapeutics based on attempts to alter the processing of the amyloid precursor protein, and the formation of senile plaques. As these approaches evolve, it will become increasingly important to determine the role of amyloid precursor protein in normal and abnormal brain. Amyloid precursor protein may have beneficial therapeutic effects that need to be preserved. Any agent designed to alter the process of amyloid precursor protein would be designed to alter the course of Alzheimer's disease. Methodological problems abound in clinical studies designed to show the ability of a therapeutic agent to slow the progress of a degenerative brain disease. These difficulties and possible solutions are explored.