Purpose : To evaluate the frequency, distribution, appearance, and clinical outcome of brainstem injury, as seen on MR, in a prospective study of patients with traumatic perimesencephalic subarac-hnoid hemorrhage (pSAH)seen on initial CT scan. Materials and Methods : MR images were prospectively obtained in 38 patients with head injury who on initial CT scans showed pSAH. To identify the amount and location of pSAH, the CT scans of all patientd, and MRI findings were evaluated according to the presence, location and signal intensity of brainstem injury, and other combined intracranial injuries. Initial Glasgow coma scale(GCS) and Glasgow outcome scale(GOS),as noted on clinincal records, were reviewed.Results : Brainstem injury was demonstrated on MR images in 30patients(79%). The majority of these lesions (76.7%) were located in the dorsolateral portion, and nonhemorrhagic lesions were more frequent(70%) than hemorrhagic. In patients with brainstem injury, as seen on MR imaging, the GOS score was worse, especially in those with combined diffuse axonal injury in the corpus callosum and cerebral white matter. The location and amount of pSAH seen on CT was not related with brainstem injury or clinical outcome. Conclusion: The presence of pSAH in patients with acute head trauma, as seen on CT was thought to be an indicator of brainstem injury, and MR imaging was necessary. If such injury was identified on MRI, this was predictive of a worse clinical outcome.