Research centers and institutes are a common mechanism to organize and facilitate biomedical research at medical schools and universities. The authors report the results of a study on the size, scope, and range of activities of 604 research centers and institutes at research-intensive U.S. medical schools and their parent universities. Centers and institutes with primary missions of patient care, education, or outreach were not included. The findings indicate that, in addition to research, centers and institutes are involved in a range of activities, including education, service, and technology transfer. The centers and institutes the authors studied were more interdisciplinary than those included in previous studies on this topic. Most research centers and institutes did not have authority comparable to academic departments. Only 22% of centers directly appointed faculty members, and most center directors reported to a medical school dean or a department chair. A small group of centers and institutes ("power centers"), however, reported to a university president or provost, and may have considerable power and influence in academic decision making and resource allocation. Two main types of centers and institutes emerge from this research. The first type, which includes the vast of majority of centers, is modest in its scope and marginal in its influence. The second type--with greater amounts of funding, larger staffs, and direct access to institutional decisionmakers--may have a more significant role in the organization and governance of the medical school and university and in the ways that researchers interact within and across academic divisions.