We surveyed a national sample of 879 physicians practicing in adult intensive care units in the United States, in order to determine their practices with regard to limiting life-sustaining medical treatment, and particularly their decisions to continue or forgo life support without the consent or against the wishes of patients or surrogates. Virtually all of the respondents (96%) have withheld and withdrawn life-sustaining medical treatment on the expectation of a patient's death, and most do so frequently in the course of a year. Many physicians continue life-sustaining treatment despite patient or surrogate wishes that it be discontinued (34%), and many unilaterally withhold (83%) or withdraw (82%) life-sustaining treatment that they judge to be futile. Some of these decisions are made without the knowledge or consent of patients or their surrogates, and some are made over their objections. We conclude that physicians do not reflexively accept requests by patients or surrogates to limit or continue life-sustaining treatment, but place these requests alongside a collection of other factors, including assessments of prognosis and perceptions of other ethical, legal, and policy guidelines. While debate continues about the ethical and legal foundations of medical futility, our results suggest that most critical care physicians are incorporating some concept of medical futility into decision making at the bedside.