Spurred by demands for data from employer-purchasers and accreditation agencies and the adoption of strategies for disease management and outcome-based quality assurance, managed care organizations have recognized the need for rapid, convenient access to clinical information. Large investments in administrative and clinical data systems have also produced unprecedented opportunities for research on health care and epidemiology in large, defined populations. There is a long history of contributions to research by investigators who are based in the older nonprofit group and staff models of health maintenance organizations (HMOs). Many of these organizations maintain research units that are primarily funded by outside sources. Research includes descriptive and etiologic studies of epidemiology, randomized and observational studies of the effectiveness of treatment regimens, studies of disease costs and estimation of cost-effectiveness, investigations of risk predictions in populations, of risk and changes in organizational behavior, and evaluations of interventions to alter physician and patient behavior. The work is often conducted in collaboration with academic researchers. The HMO Research Network has recently been established to foster a scientific exchange among HMO-based researchers. As managed care organizations come to provide health care coverage to most U.S. citizens, research conducted by these organizations increasingly overlaps with public health research. Collaboration between HMO-based research centers and researchers from academia and government will undoubtedly continue to increase.