A conference was held in Houston, Texas, on October 8-9, 1991, to develop recommendations for outcome measures for clinical trials in traumatic brain injury. Participants, all experts in this area, discussed and agreed on treatments for patients with severe brain injury (Glasgow Coma Score [GCS] < or = 8) and moderate brain injury (GCS, 9-12). A parallel trial design was recommended rather than a factorial, sequential, or crossover design. It was agreed that stratifying randomization based on motor score alone or on a combination of motor score and age would result in increased power. Acute stage measurements, such as cerebral blood flow, cerebrospinal fluid biochemistry, and evoked potentials, were recommended only when they satisfied a specific hypothesis. Functional outcome measures were recommended as the primary outcome measure for severe brain injury (GCS, 3-8). Either the Glasgow Outcome Scale or Disability Rating Scale, measured at 6 months after injury, were recommended as the primary outcome measure for severe brain injury (GCS, < or = 8). For patients with moderately severe brain injury (GCS, 9-12), the Disability Rating Scale at 3 months after injury was recommended as the primary outcome measure. The Neurobehavioral Rating Scale appears to be a satisfactory instrument for measuring behavioral changes. Specific neuropsychological measures were recommended as supplementary outcome measures for both severe and moderate brain injury, consistent with a 1.5-hour period available for testing.