Trust, the expectation that institutions and professionals will act in one's interests, contributes to the effectiveness of medical care. With the rapid privatization of medical care and the growth of managed care, trust may be diminished. Five important aspects of trust are examined: technical and interpersonal competence, physician agency, physician control, confidentiality, and open communication and disclosure. In each case, changing health care arrangements increase the risks of trusting and encourage regulatory interventions that substitute for some aspects of trust. With the increased size and centralization of health care plans, inevitable errors are attributed to health plans rather than to failures of individual judgment. Such generalized criticisms exacerbate distrust and encourage micromanagement of medical care processes.