The use of diagnostic tests, especially imaging studies, varies markedly across the United States-with higher costs but no better patient outcomes associated with the highest-use regions. A proposed new model of the health care system draws on an analogy with the ecosystem to explain the geographic variations in physician test ordering. This framework emphasizes the adaptability and interdependence of the components of the system. Patients and physicians are influenced by the health care organizations in their community, including the practice site in which the physician works, local hospitals, malpractice lawyers, and imaging centers. These are in turn influenced by institutions in society at large, including the media, health care plans, and the government. Further adaptations to the explanatory model account for the psychologic and sociologic aspects of physician behavior. Understanding the medical ecoculture is essential for effective health care reform because widely touted changes, such as the introduction of an electronic medical record or comparative effectiveness studies, do not address the adaptability and interdependence that characterize the medical ecoculture.