The purpose of the study is to determine if treating head-injured patients with magnesium sulfate will improve medical, mental, and psychological recovery. In particular, the study will assess each patient's ability to return to daily life, live independently, and return to work or school as done before the head injury occurred. The study will also assess magnesium sulfate's ability to reduce the risk of developing seizures (epilepsy) as well as to improve survival rates after a traumatic brain injury. Patients on the study are assigned randomly (by chance) to either the magnesium sulfate group or the group which gets a placebo. This means they have an equal chance of being in either group. Before the first dose is given, two teaspoons worth of blood are drawn from a vein in the arm. The first dose of magnesium sulfate is 1meq/kg given intravenously within 8 hours of injury. Then a five day continuous intravenous infusion of magnesium sulfate 0.24meq/kg per hour is begun. Daily magnesium levels are checked and the dose changed in order to keep the Magnesium blood level at approximately 4meq/L. If the person does not receive magnesium sulfate, he receives a placebo which looks just like magnesium sulfate but contains no active medication. If the person leaves the hospital before the five days are over, the magnesium or placebo is stopped. Patients on the study will receive a brief exam in person or over the phone at one and three months after the injury to determine whether they have had any seizures and to evaluate how they are functioning and recovering from their head injury. Each evaluation will last about one hour. At six months after the injury, they will have full neuropsychological and psychosocial evaluations done at Harborview. These tests will take about five hours to complete and include tests of vocabulary, problem solving, and coordination. There will be questions about how the injury has affected the way they feel and interact socially. For example, there will be questions about their ability to work, manage personal affairs, what their moods are like, and how anger is handled.