There appears to be a high incidence of fever after brain injury. The most common cause for fever is infection. The incidence of fever occurring as a result of hypothalamic thermoregulatory dysfunction after brain injury is less clear. This study retrospectively reviewed the charts of 286 subjects with brain injuries. Subject subpopulations were divided into traumatic brain injuries, anoxic brain injuries, and brain injuries resulting from aneurysmal subarachnoid hemorrhage. Fever events were described as any core temperature greater than 99.9 degrees F. Most subjects suffered a severe brain injury and had an average acute hospital length of stay ranging from 35.4 to 60 days. The average rehabilitation length of stay ranged from 38.4 to 45.1 days. Twenty-four percent of subjects experienced fevers, with each of the populations having similar occurrence rates. Unexplained fever events were found in the traumatic brain injury (7%) and aneurysmal subarachnoid hemorrhage (8%) subpopulations only. No unexplained fever event was associated with a temperature greater than 100.8 degrees F.