Severe traumatic brain injury (TBI) remains a critical problem in the United States and throughout the world. Advances in critical care of the TBI patient has resulted in improved outcome. However, despite these efforts, half of the patients with severe brain injury die or are left with severe disability. Thus, the need for a pharmaceutical agent to blunt the cascade of neurotoxins released with mechanical head trauma and improve outcome is critically important. One of these neurotoxins is endothelin, a potent vasoactive peptide, which is considered to play a major role in TBI, particularly with subarachnoid hemorrhage and concomitant vasospasm. However, relatively little is known regarding the up-regulation of this toxin after head injury. Thus, the purpose of this observational study is to document the levels of endothelin in cerebrospinal fluid and plasma of patients with brain injury over a 14 day period. Aliquots of CSF and blood will be obtained at 12 hour intervals as long as a ventriculostomy for sampling of CSF is required for routine management. Additional information regarding the severity of injury, CT pathology, intracranial pressure, neurologic assessment, CSF biomarkers, and transcranial doppler ultrasound will also be collected. The study will be implemented among 12 centers,8 in the U.S.A and 3 in Europe and 1 in Canada, providing an average of approximately 5 patients per center over a period of 7 months. The study may be extended based on an interim analysis of endothelin levels at various injury severity levels. Data will be collected and coordinated by the American Brain Injury Consortium utilizing a web-based entry system. Following the completion of accrual a final report will be assembled by the ABIC which will serve as the basis for scientific publication and provide reference data for future development of clinical trials utilizing the endothelin system.