OBJECTIVES: Autonomic nerve system dysfunction has been reported in patients following traumatic brain injury (TBI). However, studies providing direct evidence of a link between the two have been scarce. The purpose of the present study was to evaluate cardiac autonomic modulation in post-acute ambulatory outpatients with TBI using an analysis of heart rate variability (HRV). METHODS: Twenty-one patients with TBI and 21 age- and gender-matched normal controls were evaluated. The HRV assessments were performed using conventional time and frequency domain analyses as well as non-linear complexity analysis. RESULTS: In the patient group, standard deviation values of the RR intervals (SDNN), the square root of the mean squared differences of successive normal sinus intervals (RMSSD), total successive RR interval difference (TSRD), and the very low frequency (VLF) power spectral parameter were significantly lower than in the control group. The patients with moderate severity of TBI exhibited a significantly lower value of VLF than those with mild severity. The severity of anxiety symptoms had a tendency to be correlated with the mean heart rate and the ratio of low-to-high frequency spectral power. CONCLUSION: The results of the present study provide evidence that TBI is associated with reduced HRV, suggesting the critical involvement of central autonomic structures in the pathogenesis of TBI. The HRV measures may serve as an index for monitoring aspects of autonomic function in patients with TBI.