Physicians are frequently unaware of patient preferences for end-of-life care. Identifying and exploring barriers to patient-physician communication about end-of-life issues may help guide physicians and their patients toward more effective discussions. To examine correlates and associated outcomes of patient communication and patient preferences for communication with physicians about cardiopulmonary resuscitation and prolonged mechanical ventilation. Prospective cohort study. Five tertiary care hospitals. 1832 (85%) of 2162 eligible patients completed interviews. Surveys of patient characteristics and preferences for end-of-life care; perceptions of prognosis, decision making, and quality of life; and patient preferences for communication with physicians about end-of-life decisions. Fewer than one fourth (23%) of seriously ill patients had discussed preferences for cardiopulmonary resuscitation with their physicians. Of patients who had not discussed their preferences for resuscitation, 58% were not interested in doing so. Of patients who had not discussed and did not want to discuss their preferences, 25% did not want resuscitation. In multivariable analyses, patient factors independently associated with not wanting to discuss preferences for cardiopulmonary resuscitation included being of an ethnicity other than black (adjusted odds ratio [OR], 1.48 [95% CI, 1.10 to 1.99), not having an advance directive (OR, 1.35 [CI, 1.04 to 1.76]), estimating an excellent prognosis (OR, 1.72 [CI, 1.32 to 2.59]), reporting fair to excellent quality of life (OR, 1.36 [CI, 1.05 to 1.76]), and not desiring active involvement in medical decisions (OR, 1.33 [CI, 1.07 to 1.65]). Factors independently associated with wanting to discuss preferences for resuscitation but not doing so included being black (OR, 1.53 [CI, 1.11 to 2.11]) and being younger (OR, 1.14 per 10-year interval younger [CI, 1.04 to 1.25]). Among seriously ill hospitalized adults, communication about preferences for cardiopulmonary resuscitation is uncommon. A majority of patients who have not discussed preferences for end-of-life care do not want to do so. For patients who do not want to discuss their preferences, as well as patients with an unmet need for such discussions, failure to discuss preferences for cardiopulmonary resuscitation and mechanical ventilation may result in unwanted interventions.