While most would agree that mild traumatic brain injury (TBI) is associated with early neuropsychological problems, disagreement exists regarding their persistence and whether they are the cause of the disabilities experienced by some people. The aim of this study was to examine how the criteria used to define mild TBI and how the pre-injury characteristics of people affect their neuropsychological outcome. A total of 157 unselected hospitalized cases with Glasgow Coma Scale scores of 13-15 and 109 trauma controls were prospectively recruited and administered a number of cognitive measures at 1 month and 12 months after injury. The results indicated early impairments that decreased with time and the stringency of the definition of 'mild' TBI. The contribution of demographics was usually significant and often stronger than the mild TBI effect. Subtle variation of the demographics of the brain injured or the comparison subjects can be sufficient to mimic or mask mild brain injury effects.