The traditional exchange of medical expertise between physicians for patient benefit has been accomplished by referral. Physicians have traditionally decided when and to whom to refer patients. Health care "systems" now dominate medical practice, and their formats can alter spontaneous collegial interaction in referral. Institutional programs now pursue patient referrals as part of a marketing strategy to attract new patients who then become attached to the institution, rather than to a physician. Referral behavior can affect a physician's personal income in prepaid insurance programs where referrals are discouraged. The referring physician may bear legal liability for actions of the consultant. New practice arrangements and affiliations may place physicians in financial conflict-of-interest situations, challenge ethical commitments, and add new moral responsibility.