Using the ethical concepts of co-fiduciary responsibility in patient care and of preventive ethics, this article provides an ethical framework to guide physician and lay leaders of accountable care organizations. The concept of co-fiduciary responsibility is based on the ethical concept of medicine as a profession, which was introduced into the history of medical ethics in the 18th century. Co-fiduciary responsibility applies to everyone who influences the processes of patient care: physicians, organizational leaders, patients, and patients' surrogates. A preventive ethics approach to co-fiduciary responsibility requires leaders of accountable care organizations to create organizational cultures of fiduciary professionalism that implement and support the following: improving quality based on candor and accountability, reasserting the physician's professional role in the informed consent process, and constraining patients' and surrogates' autonomy. Sustainable organizational cultures of fiduciary professionalism will require commitment of organizational resources and constant vigilance over the intellectual and moral integrity of organizational culture.