Workforce planning in an era of health care reform is a challenge as both delivery systems and patient demographics change. Current workforce projections are based on a future health care system that is either an identified "ideal" or a modified version of the existing system. The desire to plan for such an "ideal system," however, may threaten access to necessary services if it does not come to fruition or is based on theoretical rather than empirical data.Historically, workforce planning that concentrated only on an "ideal system" has been centered on incorrect assumptions. Two examples of such failures presented in the 1980s when the Graduate Medical Education National Advisory Committee recommended a decrease in the physician workforce on the basis of predetermined "necessary and appropriate" services and in the 1990s, when planners expected managed care and health maintenance organizations to completely overhaul the existing health care system. Neither accounted for human behavior, demographic changes, and actual demand for health care services, leaving the nation ill-prepared to care for an aging population with chronic disease.In this article, the authors argue that workforce planning should begin with the current system and make adjustments based on empirical data that accurately reflect current trends. Actual health care use patterns will become evident as systemic changes are realized-or not-over time. No single approach will solve the looming physician shortage, but the danger of planning only for an ideal system is being unprepared for the actual needs of the population.