Hypersomnia following head injury is quite common, but the diagnosis of posttraumatic narcolepsy has tended to court controversy to the extent that the existence of this disorder has been questioned. The recent discovery of the hypocretins (orexins) and the subsequent confirmation that their deficiency causes narcolepsy have shed light on the pathophysiology of narcolepsy. Damage to or dysfunction of the hypothalamic cells that produce hypocretin (orexin), from whatever cause, can give rise to narcolepsy. We have conducted a review of the literature on the subject and have included only those cases that have been confirmed by polysomnography. There are approximately 20 polysomnography-confirmed cases of posttraumatic narcolepsy published in the literature, to which we add a further 2 cases. The results suggest that patients with posttraumatic narcolepsy are a heterogeneous group with respect to clinical presentation, nature and severity of injury, time from index event to symptom onset, and HLA type. There is some evidence that this type of narcolepsy may follow a progressive course, suggesting that the index event sets off a cascade sequence that may involve the hypocretin system. Further research in this area is required to answer these hypotheses.