INTRODUCTION: Acquired brain injury (ABI) cause neural deficits. In addition to motor and cognitive deficits, the autonomic nervous system may be affected. This has been shown for neurorehabilitation patients with traumatic brain injury (TBI) by means of reduced heart rate variability (HRV). It was hypothesized that patient groups with other ABI aetiology (mainly stroke, subarachnoid haemorrhage and anoxia) would also present reduced HRV. METHODS: Patients consecutively admitted and severely ABI injured were considered for HRV measurements. HRV was extracted as a mean of four 5-minute ECG recordings at 6 pm, 10 pm, 2 am and 6 am the following day (scheduled resting periods). One 5-minute HRV recording from a sex- and age-matched group of healthy volunteers constituted control data. Standard deviation of normal-to-normal intervals (SDNN) and low frequency (LF) were primary HRV variables. RESULTS: Of 71 admitted patients, HRV was extracted from 49 patients. Patient SDNN and LF were reduced compared to controls (SDNN: 13 ms (CI = [10.8; 15.3]) vs 40.3 ms (CI = [36.6; 44.2]), p < 0.0001; LF: 29.4 ms(2) (CI = [19.8; 43.7]) vs 518 ms(2) (CI = [419; 639]), p < 0.0001). HRV appeared identical across ABI aetiology. CONCLUSION: It was found that HRV was considerably reduced in an heterogenic ABI patient group admitted for neurorehabilitation.