<h3>Background:</h3> Physician specialty training is associated with variations in the use of medical treatment for specific diseases. <h3>Objective:</h3> To examine whether physicians' specialties predict differences in willingness to use life-sustaining treatments. <h3>Methods:</h3> One hundred fifty-eight physicians (response rate, 85%) who cared for 378 hospitalized patients with end-stage congestive heart failure, chronic obstructive pulmonary disease, malignant neoplasms, or hepatic cirrhosis were interviewed to assess their thresholds for use of specific life-sustaining treatments. Their patients were then followed up to determine whether decisions were made to use or withhold cardiopulmonary resuscitation, ventilator support, or intensive care. Physicians' attitudes, their stated thresholds for treatment use, and their use of these treatments in daily practice were compared by specialty group. <h3>Results:</h3> Physicians recommended cardiopulmonary resuscitation and ventilator support for patients with end-stage congestive heart failure or chronic obstructive pulmonary disease if the chance for survival was at least 48%, but they required a predicted survival of at least 74% for patients with cancer. For a patient with end-stage congestive heart failure or chronic obstructive pulmonary disease, cardiologists were consistently more willing than other physicians to use life-sustaining treatments. In practice, decisions to use or withhold such treatments were made for 151 patients with end-stage diseases. Compared with other physicians, cardiologists were least likely to issue orders to withhold treatment and most likely to use life-sustaining treatments for patients they treated. Oncologists rarely used such treatments and issued orders to withhold these treatments much more often. <h3>Conclusion:</h3> Physician specialty is associated with differences in willingness to use, and in actual use of, life-sustaining treatments. (Arch Intern Med. 1996;156:785-789)