OBJECTIVE: To differentiate between traumatic brain injury (TBI) subjects with normal and elevated autonomic activity by quantifying cardiac responsivity to nociceptive stimuli and to determine the utility of heart rate variability (HRV) and event-related heart rate changes in diagnosing dysautonomia. DESIGN: Prospective cohort study. SETTING: Intensive care unit in a tertiary metropolitan trauma center. PARTICIPANTS: Adults (N=27) with TBI recruited from 79 consecutive TBI admissions comprising 16 autonomically aroused and 11 control subjects matched by age, sex, and injury severity. INTERVENTIONS: None. MAIN OUTCOME MEASURES: Immediate: pattern of autonomic changes indexed by HRV and event-related heart rate after nociceptive stimuli. Six months: length of stay, Glasgow Coma Scale, and Disability Rating Scale. RESULTS: Heart rate changes (for both HRV and event-related heart rate) were associated with the diagnostic group and 6-month outcome when evaluated pre- and poststimulus but not when evaluated at rest. When assessed on day 7 postinjury, the comparison of HRV and heart rate parameters suggested an overresponsivity to nociceptive stimuli in dysautonomic subjects. These subjects showed a 2-fold increase in mean heart rate relative to subjects with sympathetic arousal of short duration (16% vs 8%), and a 6-fold increase over nonaroused control subjects. Data suggest that post-TBI sympathetic arousal is a spectrum disorder comprising, at one end, a short-duration syndrome and, at the other end, a dramatic, severe sympathetic and motor overactivity syndrome that continued for many months postinjury and associated with a significantly worse 6-month outcome. These findings suggest that it is not the presence of reactivity per se but rather the failure of processes to control for overreactivity that contributes to dysautonomic storming. CONCLUSIONS: This study provides empirical evidence that dysautonomic subjects show overresponsiveness to afferent stimuli. Findings from this study suggest an evidence-driven revision of diagnostic criteria and a simple clinical algorithm for the improved identification of cases.