OBJECTIVE: To evaluate heart rate variability (HRV) of patients with traumatic brain injury (TBI). METHODS: By a prospective study, the HRV was assessed in 20 patients with TBI during the sub-acute period post-injury (the first test was performed at a mean time post-insult of 38 days) and a matched control. The patients were examined twice, 1 month apart. The assessment included HRV (both in time and frequency domains), GCS, length of coma, brain CT, FIM and FAM. RESULTS: A significant difference was found between patients and controls concerning HRV total power, i.e. frequencies between 0.01-0.6 Hz (high frequency p = 0.003, low frequency p = 0.013, total power p = 0.034) and for standard deviation of RR interval p = 0.011. HRV changes were related more to the timing of the evaluation than to the severity of the brain damage. CONCLUSION: HRV differed of patients with TBI and in the control group. Tendency to HRV normalization changes was detected during the first 3 months after the injury, which suggests recovery of the autonomic nervous system.