Certain disease states are characterized by disturbances in production, accumulation or clearance of protein. In Alzheimer disease, accumulation of amyloid-β (Aβ) in the brain and disease-causing mutations in amyloid precursor protein or in enzymes that produce Aβ indicate dysregulation of production or clearance of Aβ. Whether dysregulation of Aβ synthesis or clearance causes the most common form of Alzheimer disease (sporadic, >99% of cases), however, is not known. Here, we describe a method to determine the production and clearance rates of proteins within the human central nervous system (CNS). We report the first measurements of the fractional production and clearance rates of Aβ in vivo in the human CNS to be 7.6% per hour and 8.3% per hour, respectively. This method may be used to search for novel biomarkers of disease, to assess underlying differences in protein metabolism that contribute to disease and to evaluate treatments in terms of their pharmacodynamic effects on proposed disease-causing pathways.