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Introduction: The mismatch negativity (MMN) is a component of the long-latency auditory evoked potentials, that are used to check the functionality of automatic attentional processes of attentive information processing. Vegetative state diagnosis is frequent following severe traumatic brain injury (TBI). Eventually, some patients improve their condition to another one called minimal conscientious state., Aim: To evaluate the diagnostic usefulness of MMN in severe TBI patients during the subacute phase after leaving the neurological intensive care unit., Patients and Methods: We gathered MMN results from 19 patients (12 males and 7 females; 8 vegetative state and 11 minimal conscientious state) with ages between 17 and 59 years (mean: 27.3 years). The delay between TBI onset and MMN recordings were greater than 2 months (mean: 181 days). During the recording session, patients were evaluated by means of the Multisociety Task Force on Persistent Vegetative State scale., Results: All the minimal conscientious state patients (100%) showed MMN potential. In seven of the vegetative state patients (87.5%) the MMN was not found. The remaining MMN potential positive vegetative state patient improved to MCS 16 days after testing., Conclusions: MMN is a valid tool for differentiating vegetative state from MCS during the subacute phase of severe TBI. Hence it is a useful aid to the clinical evaluation. This would diminish the impact of a possible diagnostic error and its prognosis on therapeutical management, family and health costs. It can also be used for evaluating other brain disorders with altered consciousness.