In this study of patients with moderate traumatic brain injury with frontal brain contusions patients will be randomized to treatment with a single dose of either D-cycloserine or placebo given 48-72 hours after injury.

Patients will undergo intensive neurocognitive testing at 3 and 6 months after injury to assess whether treatment with a single dose of D-cycloserine improves cognitive outcome after moderate brain injury.

Inclusion Criteria:

1. TBI diagnosed by history and/or clinical examination with presenting GCS between 9 -13
2. Age between 18 and 55 years
3. Frontal cerebral contusion(s) located in the frontal cortex and/or traumatic subarachnoid hemorrhage on initial CT scan
4. Prior to randomization, patient is able to swallow an orally administered tablet or has a nasogastric tube in place per medical indication
5. Informed consent will be obtained in those patients who have regained full cognition and judgment following their injury and are fully capable of understanding the study protocol, its risks and potential benefits, and of giving informed consent for participation in the study. In any patient that has not regained full cognition and judgment, only a temporary legal guardian (apotropos) appointed by an authorized judge that has been given a full explanation of the study protocol, its risks and potential benefits will be authorized to give informed consent for participation in the study.

Exclusion Criteria:

1. Penetrating brain injury
2. Any traumatic intracranial lesion requiring neurosurgical intervention prior to time of randomization
3. Other severe systemic injuries leading to severe hypotension, hemodynamic instability following initial resuscitation (SBP < 90 mmHg), severe hypoxia.
4. Intubation lasting > 12 hours prior to randomization
5. Suspected or confirmed pregnancy or lactating women
6. Any spinal cord injury
7. Known or CT scan evidence of previous major cerebral damage
8. Any severe concomitant condition (malignancy, renal, hepatic, or major psychiatric disorder)
9. Known treatment with another investigational drug within 30 days of injury
10. Known contraindication to enteral administration of drug prior to randomization (GI bleed, ileus, severe abdominal injury, etc.)
11. Known contraindication to placement of a nasogastric tube in patients unable to swallow an orally administered tablet