Traditionally, projections of US health care demand have been based upon a combination of existing trends in usage and idealized or expected delivery system changes. For example, 1990s health care demand projections were based upon an expectation that delivery models would move toward closed, tightly managed care networks and would greatly decrease the demand for subspecialty care. Today, however, a different equation is needed on which to base such projections. Realistic workforce planning must take into account the fact that expanded access to health care, a growing and aging population, increased comorbidity, and longer life expectancy will all increase the use of health care services per capita over the next few decades—at a time when the number of physicians per capita will begin to drop. New technologies and more aggressive screening may also change the equation. Strategies to address these increasing demands on the health system must include expanded physician training.