Upon admission, 17 of 223 (8%) consecutive patients with severe head injury exhibited a flaccid, wholly unresponsive motor examination. In this study alcoholic intoxication neither caused depressed motor responsiveness in head-injured patients with high serum ethanol levels nor accounted for the motor examination in those exhibiting the flaccid state. Flaccidity was attributed principally to impaired ventilation in 4 patients, a major intracranial mass in 12. and a spinal cord injury in 1. Compared to the larger group of head-injured patients, the flaccid patients had a significantly greater incidence of hypercapnia (P < 0.001), acidosis (P < 0.01), and both elevated and uncontrollable intracranial pressure (ICP) (P< 0.001). These findings and the high mortality rate (76%) in this study suggest that the magnitude of respiratory complications and the severity of mechanical brain injury are greater in flaccid patients. The flaccid patients undergoing surgical decompression for major intracranial mass lesions (11 cases) have all died and. although still small in number, this group may represent an important subset with a poor prognosis. Nonetheless, a protocol that encourages rapid radiological and electrophysiological assessment and vigorous surgical and ICP management until the probable cause of flaccidity is identified and treated has benefit. The flaccid state was reversed and a good recovery was attained after the restoration of blood pressure and/or ventilation in 2 patients who appeared to have sustained a very grave head injury. In another patient, absent somatosensory evoked potentials greatly facilitated the diagnosis of a spinal subdural hematoma. This program of prompt diagnosis and intense therapy did not result in a protracted course or undue numbers of severely braindamaged survivors.