Most physicians in the United States have paper medical records — the sort that doctors have kept for generations. A minority have electronic records that provide, at a minimum, tools for writing progress notes and prescriptions, ordering laboratory and imaging tests, and viewing test results (see line graph).¹ Yet electronic health data are poised for an online transformation that is being catalyzed by Dossia (a nonprofit consortium of major employers), Google Health, Microsoft HealthVault, and other Web services that are seeking expanded roles in the $2.1 trillion U.S. health care system.

Online repositories will allow patients to store, retrieve, manage, and . . .