Forty patients with closed head trauma were evaluated prospectively with CT and intermediate-field-strength MR imaging to compare the diagnostic efficacies of the two techniques. Traumatic lesions were detected in 38 patients. The severity of injury, as determined by the Glascow Coma Scale, ranged from 3 to 14. The sensitivities of CT and MR were calculated for all subgroups of lesions: (1) hemorrhagic and nonhemorrhagic intraaxial lesions (diffuse axonal injury, cortical contusion, subcortical gray-matter injury, primary brainstem injury); (2) extraaxial hematomas (subdural, epidural); and (3) diffuse hemorrhage (subarachnoid, intraventricular). CT and MR (T1- and T2-weighted) studies were both highly and comparably sensitive in the detection of hemorrhagic intraaxial lesions. MR scans, however, were much more sensitive in detecting nonhemorrhagic lesions. cortical contusions and diffuse axonal injury constituted 91.9% of all intraaxial lesions. The sensitivities of the imaging techniques for this combined group of lesions were (1) nonhemorrhagic lesions (CT = 17.7%, T1-weighted MR = 67.6%, T2-weighted MR = 93.3%); (2) hemorrhagic lesions (CT = 89.8%, T1-weighted MR = 87.1%, T2-weighted MR = 92.5%). MR was also significantly better in detecting brainstem lesions (CT = 9.1%, T1-weighted MR = 81.8%, T2-weighted MR = 72.7%). The sensitivities of the diagnostic studies in the detection of extraaxial hematomas were CT = 73.2%, T1-weighted MR = 97.6%, T2-weighted MR = 90.5%). Intraventricular hemorrhage was consistently seen with all three imaging studies, but subarachnoid hemorrhage was detected much more frequently with CT. In summary, MR has clear advantages over CT in evaluating closed head trauma. Although its sensitivity in detecting hemorrhagic lesions is similar to that of CT, it is much better than CT in detecting nonhemorrhagic lesions, which are more prevalent. MR is more useful than CT in classifying primary and secondary forms of injury and directing treatment. CT's one advantage over MR is its ability to more rapidly assess unstable patients who may need surgery.