A 2 1/4-year prospective study of children suffering head injury is described. Three groups of children were studied: (a) 31 children with 'severe' head injuries resulting in a post-traumatic amnesia (PTA) of at least 7 days; (b) an individually matched control group of 28 children with hospital-treated orthopaedic injuries; and (c) 29 children with 'mild' head injuries resulting in a PTA exceeding 1 hour but less than 1 week. A retrospective assessment of the children's pre-accident behaviour was obtained by parental interview and teacher questionnaire immediately after the accident and before the behavioural sequelae of the injury could be known. Further psychiatric assessments were undertaken 4 months, 1 year and 2 1/4 years after the initial injury. The mild head injury group showed a raised level of behavioural disturbance before the accident but no increase thereafter. It was concluded that head injuries resulting in a PTA of less than 1 week did not appreciably increase the psychiatric risk. By contrast, there was a marked increase in psychiatric disorders following severe head injury. The high rate of new disorders in children with severe head injuries who were without disorder before the accident, together with the finding of a dose-response relationship with the severity of brain injury, indicated a causal relationship. However, the development of psychiatric disorders in children with severe head injuries was also influenced by the children's pre-accident behaviour, their intellectual level, and their psychosocial circumstances. With the exception of social disinhibition and a slight tendency for the disorders to show greater persistence over time, the disorders attributable to head injury showed no specific features.