PURPOSE: To evaluate the frequency, distribution, appearance, and clinical outcome of brainstem injury, asseen 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 headinjury who on initial CT scans showed pSAH. To identify the amount and location of pSAH, the CT scans of allpatientd, and MRI findings were evaluated according to the presence, location and signal intensity of brainsteminjury, 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 nonhemorrhagiclesions were more frequent(70%) than hemorrhagic. In patients with brainstem injury, as seen on MR imaging, theGOS score was worse, especially in those with combined diffuse axonal injury in the corpus callosum and cerebralwhite matter. The location and amount of pSAH seen on CT was not related with brainstem injury or clinicaloutcome. CONCLUSION: The presence of pSAH in patients with acute head trauma, as seen on CT was thought to be anindicator of brainstem injury, and MR imaging was necessary. If such injury was identified on MRI, this waspredictive of a worse clinical outcome.