Integrated care systems have unique advantages for conducting research. The HMO Research Network (HMORN) includes research centers associated with 13 large integrated care systems whose research focuses on improving health and health care delivery using the extraordinary platform provided by these health systems. We conducted literature reviews and surveys and interviews with directors of HMORN research centers, research investigators, and selected support staff in order to identify the characteristics of the research in HMORN centers and to present examples of how this research has affected health and health policy. The 13 HMORN member health systems deliver health care to 13 million people. HMORN research centers have access to large, defined populations, comprehensive medical information, extensive computerized data systems and to medical care delivery systems that offer extraordinary research opportunities. HMORN centers publish about 1200 scientific articles each year and received about $180 million in external research funding in 2002, most of it from NIH, CDC, and other federal sources. More than 2000 research studies are currently underway at these centers, which employ approximately 1500 persons in the research activities. HMORN research centers have had a profound impact on health policy and care. New technologies are steadily expanding the research capacities of these research groups. Increased collaboration between academic and HMO researchers would enhance the work of both.