Magnetic resonance imaging (MRI) is an invaluable tool in the evaluation of intracranial and spinal disorders. However, because of various technical limitations, the use of MRI in head-injured patients has yet to be fully explored. With its precise anatomical sensitivity, MRI may be useful in severely head-injured patients in whom computed tomographic (CT) scans fail to demonstrate an anatomical substrate for the degree of coma. In this regard, a prospective study in severely head-injured patients was undertaken. Twenty-four patients with Glasgow coma scores of 7 or less who had minimal or no CT abnormalities and normal intracranial pressures underwent MRI as soon as their medical conditions allowed. In all 24, MRI demonstrated lesions that were not evident on repeated CT scans--suspected white matter shear injuries or contusions in 10, brain stem injuries in 5, diffuse white matter injury in 5, and subdural hematoma in 4. None of the 19 patients with the most widespread MRI abnormalities or the presence of brain stem injuries made any significant neurological recovery. Various prognostic indicators of the outcome of acute posttraumatic coma are continuously being developed. In this group of patients, the MRI scan is the most sensitive measure. Significant MRI abnormalities with normal CT scans and intracranial pressures were universally associated with vegetative outcome in this series. As we gain experience imaging neurotrauma, MRI may form the basis for a better in vivo understanding of the substrate for and natural history of traumatically induced coma.