Establish the best time to start rehabilitation by means of scientific evidence. Observational study in patients with a diagnosis of Severe Brain Injury who received intensive inpatient rehabilitation after acute care. 1470 subjects enrolled: 651 with Traumatic Brain Injury (TBI) and 819 with Non-TBI. Male gender was prevalent in the population study, but sex distribution was not different among groups, with a prevalence of male gender in both populations. This project involved 29 rehabilitation facilities for Severe ABI. The registry was an electronic database, remained active only during the period of data collection. The patients were divided into three different categories according to the time interval from brain injury to inpatient rehabilitation admission and demographic and clinical data were collected. Etiology, time interval from injury to inpatient rehabilitation, disability severity, the presence of tracheostomy at admission to the rehabilitation facility, rehabilitation length of stay and transfer back to acute care wards because of medical, surgical or neurosurgical complications. The interval from brain injury to rehabilitation facilities admission increases along with age, brain injury severity according to DRS scores, the presence of a tracheal tube and the percentage of transfers back to acute care wards from rehabilitation facilities, because of medical, surgical or neurosurgical complications. The better recovery and more positive outcomes, reported as resulting from early rehabilitation, may be due more to less severity of brain injury and fewer complications in the acute and post-acute phase than to when the rehabilitation starts.