Academic medical centers have fulfilled several of their missions with immense success but have failed to fulfill others. They have responded only modestly to the needs of the nation's underserved rural and urban communities. The author calls on academic medical centers to take an aggressively active role in building the medical infrastructure now missing in these communities and outlines a multi-part agenda for institutional commitment. It includes developing community-based systems of primary care, outreach programs, and social supports; training professionals committed to serving isolated and poor communities; and performing research that will extend the knowledge base to include the health and social issues of the disadvantaged. (Examples are given of institutions that have pioneered these kinds of community-based activities.) To build the new infrastructure, financing must be secured (various sources are discussed), a community-based faculty must be developed, and each institution's leadership--the medical school dean, the hospital executive, and the department chairmen--must come together around a new agenda and support it materially and psychologically, making whatever changes are needed in the corporate culture. The author warns that if centers do not undertake this responsibility for the health of the underserved, a critical job will go undone, a huge opportunity will have been missed, and American society will be the poorer.