In making decisions about life-sustaining medical interventions, respect for patient autonomy has been widely advocated, yet little is known about what variables may compete with a physician's ability to honor patient requests in clinical situations. We investigated physician attitudes and behaviors about end-of-life decisions by means of a questionnaire that posed five hypothetical scenarios in which an elderly, competent, terminally ill patient made a request that, if agreed to by the physician, could result in the patient's death. We surveyed 392 physicians in Rhode Island and asked them to decide (1) whether or not they would comply with a specific patient request, (2) the justifications they used in making their decision, and (3) whether they had been approached with such a request in their clinical practices. Two hundred fifty-six physicians (65%) responded. Of the respondents, 98% agreed not to intubate the patient in the face of worsening respiratory failure. Eighty-six percent agreed to give the patient a dose of narcotics that could cause respiratory compromise and death to treat his pain adequately. Fifty-nine percent agreed, once the patient was intubated without hope of coming off the respirator, to turn the respirator off. Nine percent agreed to give the patient a prescription for an amount of sleeping pills that would be lethal if taken all at once. Only 1% agreed to give the patient a lethal injection. When they complied with patient requests, physicians cited patient autonomy as the principle most important to their decision making. Physicians who would not comply with patient requests also, paradoxically, often cited this principle but agreed with it less strongly; others cited concerns about the ethical nature of the request, legal questions, and the perception that they were "killing the patient." Sixty-five percent of respondents had been asked by patients to turn off a respirator, and 12% had been asked to administer lethal injections. Twenty-eight percent of respondents indicated that they would comply with requests for lethal injection more frequently if such an action were legal. Difficult clinical decisions regarding potentially life-prolonging measures are commonly heard in clinical practice. Physicians value the concept of patient autonomy but place it in the context of other ethical and legal concerns and do not always accept specific actions derived from this principle.