The contribution of group- and staff-model HMOs to American medicine.

Although group- and staff-model prepaid health plans were the original model of HMOs, they now represent a minority of HMOs and their enrollees. Nevertheless, these models made, and continue to make, important public contributions through their demonstration of alternative methods of delivering care and their support of population-based research on specific diseases, utilization of services, and styles of medical practice. The limited number of such plans, however, makes it difficult to ascertain whether these contributions are attributable to the type of HMO per se, with their largely nonprofit ownership, their unique organizational histories, and their key leaders, among other factors. A more comprehensive understanding of this question is crucial to assuring the continuation of the public benefits that have accrued from these models in the past.