A review of recent research addressed two questions: how common are problems of substance abuse in traumatic brain injury (TBI), and to what extent does alcohol and other drug use mediate outcome? Studies showed alcohol intoxication present in one third to one half of hospitalizations; data for other drug intoxication were not available. Nearly two thirds of rehabilitation patients may have a history of substance abuse that preceded their injuries. Intoxication was related to acute complications, longer hospital stays, and poorer discharge status; however, these relationships may have been caused by colinearity with history. History of substance abuse showed the same morbidity, and was further associated with higher mortality rates, poorer neuropsychological outcome, and greater likelihood of repeat injuries and late deterioration. The effect of history may be caused by subgroups with more severe substance abuse problems. Implications for rehabilitation are discussed, including the potential negative impact of untreated substance abuse on the ability to document efficacy of rehabilitation efforts.