We have used the rapid progression of post-traumatic uncal herniation in spite of intensive medical therapy as the indication for twist drill trephination in the emergency department. During a 54-month period, 51 trephinations were performed on 41 patients. The trephine was placed through the temporal bone ipsilateral to the dilated pupil, and the dura mater was opened to allow partial evacuation of a hematoma. All patients subsequently underwent craniotomy, autopsy, and/or cerebral computed tomography (CT). The trephination was diagnostically accurate for the absence or presence of an extracerebral hematoma in 42 of 51 trephinations (82%). In 6 of these cases the dilated pupil responded to partial hematoma evacuation by decreasing in size. In 3 of the 6 there was a marked overall improvement in neurological status subsequent to trephination. These 3 patients later recovered to an independent functional state. Only 23 of these 41 patients (56%) with herniation profiles actually had significant extracerebral hematomas. This fact emphasizes the inadvisability of taking this type of patient directly to the operating room without a diagnostic study. A rapidly performed CT scan is the obvious first choice. However, if there is any delay in obtaining this study or when uncal herniation occurs rapidly, a twist drill trephination can be of value in diagnosing the absence or presence of a treatable extracerebral hematoma. There were no complications related to this procedure in this group.