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Patients' loved ones often make end-of-life treatment decisions, but the accuracy of their substituted judgments and the factors associated with accuracy are poorly understood.To assess the accuracy of judgments made by surrogate decision makers; ascertain the beliefs, practices, and clinical and sociodemographic factors associated with accuracy of surrogates' decisions; assess the preferences of patients for life-sustaining treatments; and compare differences in accuracy across diagnoses.Cross-sectional paired interviews.Outpatient practices of three university hospitals.250 patients with terminal diagnoses of congestive heart failure, AIDS, amyotrophic lateral sclerosis, lung cancer, and chronic obstructive pulmonary disease (50 patient-surrogate pairs in each group) and 50 general medical patients and their surrogates.The accuracy of surrogate predictions was measured by using scales based on 10 potential treatments in each of three hypothetical clinical scenarios.Preferences varied according to mode of treatment and scenario. On average, surrogates made correct predictions in 66% of instances. Accuracy was better for the permanent coma scenario than for the scenarios of severe dementia or coma with a small chance of recovery (P0.001). In a binary logit model, the accuracy of substituted judgments was positively associated with the patient having spoken with the surrogate about end-of-life issues (odds ratio [OR], 1.9 [95% CI, 1.6 to 2.3]), the patient having private insurance (OR, 1.4 [CI, 1.1 to 1.7]), the surrogate's level of education (OR, 1.5 [CI, 1.2 to 1.9]), and the patient's level of education (OR, 1.7 [CI, 1.4 to 2.2]). Accuracy was negatively associated with the patient's belief that he or she would live longer than 10 years (OR, 0.6 [CI, 0.5 to 0.7]), surrogate experience with life-sustaining treatment (OR, 0.4 [CI, 0.3 to 0.5]), surrogate participation in religious services (OR, 0.67 [CI, 0.50 to 0.91]), and a diagnosis of heart failure (OR, 0.6 [CI, 0.5 to 0.8]). Age, ethnicity, marital status, religion, and advance directives were not associated with accuracy.The accuracy of substituted judgments is associated with multiple clinically apparent patient and surrogate factors. This information can help clinicians identify conditions under which substituted judgments are likely to be accurate or inaccurate and can help target populations for education designed to improve the accuracy of surrogate decision making.