The development of new organizational forms for the delivery of health and medical care in the U.S. includes health maintenance organizations (HMOs), designed to provide a set of comprehensive basic health services to a defined population for a fixed prepaid premium. As complex organizations, HMOs have the potential for limiting the autonomy of professionals working in them. This paper describes the legal requirements and organizational mechanisms under which physicians practice in HMOs and considers the potential for conflict between the organization and professional norms.