OBJECTIVE: To determine whether heart rate variability (HRV; a physiological measure of acute stress) is associated with persistent psychological distress among family members of adult intensive care unit (ICU) patients. METHODS: This prospective study investigated family members of patients admitted to a study ICU. Participants\' variability in heart rate tracings were measured by low frequency (LF)/high frequency (HF) ratio and detrended fluctuation analysis (DFA). Questionnaires were completed 3 months after enrollment to ascertain outcome rates of anxiety, depression, and post-traumatic stress disorder (PTSD). RESULTS: Ninety-nine participants were enrolled (median LF/HF ratio, 0.92 [interquartile range, 0.64-1.38]). Of 92 participants who completed the 3-month follow-up, 29 (32%) had persistent anxiety. Logistic regression showed that LF/HF ratio (odds ratio [OR] 0.85, 95% confidence interval [CI] 0.43, 1.53) was not associated with 3-month outcomes. In an exploratory analysis, DFA alpha (OR 0.93, 95% CI 0.87, 0.99), alpha(1) (OR 0.97, 95% CI 0.94, 0.99), and alpha(2) (OR 0.94, 95% CI 0.88, 0.99) scaling components were associated with PTSD development. CONCLUSION: Almost one-third of family members experienced anxiety at three months after enrollment. HRV, measured by LF/HF ratio, was not a predictor of psychologic distress, however, exploratory analyses indicated that DFA may be associated with PTSD outcomes.