The American medical care system falls to provide effective prevention services even though some prevention services are among the most cost-effective medical procedures available. Many prevention services are routinely delivered in inefficient or ineffective ways, and new technologies may be widely and aggressively implemented despite serious doubts about their efficacy and cost-effectiveness. The barriers to effective prevention services result from conceptual limitations in our model of medical care systems, particularly the lack of a population-based perspective. A change in paradigm is needed before reforms in our health care system can improve health without bankrupting the nation.