In 100 patients intracranial exploration was done soon after severe head injury when signs of transtentorial herniation were present. A third of patients had extracerebral hematomas shown on initial burr hole examination. An additional 18 percent had parenchymal clots of clinical significance disclosed only by angiography after burr hole exploration or at autopsy. Half the patients in whom exploration was done did not have intracranial hematomas of sufficient size to warrant an operative approach. Detailed neurological examinations soon after injury and the availability of more rapid diagnostic tools may improve the accuracy of early diagnosis and allow more specific prognostication in patients with severe head injury.