ABSTRACT: Background:Treatment decisions commonly have to be made in intensive care units (ICUs). These decisions are difficult for surrogate decision makers and often lead to decisional conflict, psychological distress, and treatments misaligned with patient preferences. Objective:To determine whether a decision aid about prolonged mechanical ventilation improved prognostic concordance between surrogate decision makers and clinicians compared with a usual care control. Design:Multicenter, parallel, randomized, clinical trial. (ClinicalTrials.gov: NCT01751061). Setting:13 medical and surgical ICUs at 5 hospitals. Participants:Adult patients receiving prolonged mechanical ventilation and their surrogates, ICU physicians, and ICU nurses. Intervention:A Web-based decision aid provided personalized prognostic estimates, explained treatment options, and interactively clarified patient values to inform a family meeting. The control group received information according to usual care practices followed by a family meeting. Measurements:The primary outcome was improved concordance on 1-year survival estimates, measured with the clinician-surrogate concordance scale (range, 0 to 100 percentage points; higher scores indicate more discordance). Secondary and additional outcomes assessed the experiences of surrogates (psychological distress, decisional conflict, and quality of communication) and patients (length of stay and 6-month mortality). Outcomes assessors were blinded to group allocation. Results:The study enrolled 277 patients, 416 surrogates, and 427 clinicians. Concordance improvement did not differ between intervention and control groups (mean difference in score change from baseline, -1.7 percentage points [95% CI, -8.3 to 4.8 percentage points]; P = 0.60). Surrogates' postintervention estimates of patients' 1-year prognoses did not differ between intervention and control groups (median, 86.0% [interquartile range {IQR}, 50.0%] vs. 92.5% [IQR, 47.0%]; P = 0.23) and were substantially more optimistic than results of a validated prediction model (median, 56.0% [IQR, 43.0%]) and physician estimates (median, 50.0% [IQR, 55.5%]). Eighty-two intervention surrogates (43%) favored a treatment option that was more aggressive than their report of patient preferences. Although intervention surrogates had greater reduction in decisional conflict than control surrogates (mean difference in change from baseline, 0.4 points [CI, 0.0 to 0.7 points]; P = 0.041), other surrogate and patient outcomes did not differ. Limitation:Contamination among clinicians could have biased results toward the null hypothesis. Conclusion:A decision aid about prolonged mechanical ventilation did not improve prognostic concordance between clinicians and surrogates, reduce psychological distress among surrogates, or alter clinical outcomes. Decision support in acute care settings may require greater individualized attention for both the cognitive and affective challenges of decision making. Primary Funding Source:National Institutes of Health.