Structure and performance of health maintenance organizations: a review.

During the past decade, the number of and enrollment in health maintenance organizations (HMOs) have grown dramatically. In 1980, 236 HMOs served 9 million members. By 1989, there were 591 HMOs with over 34 million enrollees. New HMOs are very different in organizational structure and arrangements than the HMOs that were operating in the 1970s, and the health care markets they serve also have changed substantially with the increasing supply of physicians and declining hospital admissions. Consequently, the accepted research findings on HMO performance in the 1970s may have only limited usefulness in understanding the role of HMOs and their effect on today's market for health services. This is of particular concern as the Health Care Financing Administration considers the further expansion of managed care options available to Medicare and Medicaid beneficiaries. In this article, the author reviews evidence on the relationship between HMO organizational arrangements and performance, and the trends within the HMO industry toward new organizational structures. The implications for Medicare and Medicaid risk contracting are also examined.