This study aimed to investigate outcome in adults with mild traumatic brain injury (TBI) at 1 week and 3 months postinjury and to identify factors associated with persisting problems. A total of 84 adults with mild TBI were compared with 53 adults with other minor injuries as controls in terms of postconcussional symptomatology, behavior, and cognitive performance at 1 week and 3 months postinjury. At 1 week postinjury, adults with mild TBI were reporting symptoms, particularly headaches, dizziness, fatigue, visual disturbance, and memory difficulties. They exhibited slowing of information processing on neuropsychological measures, namely the WAIS-R Digit Symbol subtest and the Speed of Comprehension Test. By 3 months postinjury, the symptoms reported at 1 week had largely resolved, and no impairments were evident on neuropsychological measures. However, there was a subgroup of 24% of participants who were still suffering many symptoms, who were highly distressed, and whose lives were still significantly disrupted. These individuals did not have longer posttraumatic amnesia (PTA) duration. They were more likely to have a history of previous head injury, neurological or psychiatric problems, to be students, females, and to have been injured in a motor vehicle accident. The majority were showing significant levels of psychopathology. A range of factors, other than those directly reflecting the severity of injury, appear to be associated with outcome following mild TBI.