It has been argued that big data will enable efficiencies and accountability in health care.1,2However, to date, other industries have been far more successful at obtaining value from large-scale integration and analysis of heterogeneous data sources. What these industries have figured out is that big data becomes transformative when disparate data sets can be linked at the individual person level. In contrast, big biomedical data are scattered across institutions and intentionally isolated to protect patient privacy. Both technical and social challenges to linking these data must be addressed before big biomedical data can have their full influence on health care. It is this linkage challenge that we address in this Viewpoint./.../