Objective: To determine whether psychiatric illness is a risk factor for subsequent traumatic brain injury (TBI). Methods: Case control study in a large staff model health maintenance organisation in western Washington State. Patients with TBI, determined by International classification of diseases, 9th revision, clinical modification (ICD-9-CM) diagnoses, were 1440 health plan members who had TBI diagnosed in 1993 and who had been enrolled in the previous year, during which no TBI was ascertained. Three health plan members were randomly selected as control subjects, matched by age, sex, and reference date. Psychiatric illness in the year before the TBI reference date was determined by using computerised records of ICD-9-CM diagnoses, psychiatric medication prescriptions, and utilisation of a psychiatric service. Results: For those with a psychiatric diagnosis in the year before the reference date, the adjusted relative risk for TBI was 1.7 (95% confidence interval (CI) 1.4 to 2.0) compared with those without a psychiatric diagnosis. Patients who had filled a psychiatric medication prescription had an adjusted relative risk for TBI of 1.6 (95% CI 1.2 to 2.1) compared with those who had not filled a psychiatric medication prescription. Patients who had utilised psychiatric services had an adjusted relative risk for TBI of 1.3 (95% CI 1.0 to 1.6) compared with those who had not utilised psychiatric services. The adjusted relative risk for TBI for patients with psychiatric illness determined by any of the three psychiatric indicators was 1.6 (95% CI 1.4 to 1.9) compared with those without any psychiatric indicator. Conclusion: Psychiatric illness appears to be associated with an increased risk for TBI.T he most common mechanisms of traumatic brain injury (TBI) are motor vehicle collisions, falls, sports and recreational injuries, and assaults.1-5 The known risk factors for sustaining a TBI are age 15-24 years (with smaller peaks in the > 60 and < 5 year age groups), male sex, increased blood alcohol concentration, and lower socioeconomic status.6 7 The identification of modifiable risk factors and of high risk populations has recently been prioritised by the Department of Health and Human Services' Interagency Head Injury Task Force in the United States 8 and may lead to prevention programs that help to decrease the incidence of TBI and associated disability and mortality. Although mild cases constitute the vast majority of TBI and may result in prolonged disability, 9 relatively little is known about the risk factors for mild TBI.While it is recognised that people are more likely to suffer from psychiatric illness following TBI, 10 it is less clear whether those who sustain a TBI are more likely to have had a psychiatric illness before their injury. Previous studies suggest that psychiatric illness may be a risk factor for traumatic accidents but they did not specifically examine TBI as an outcome. [11][12][13][14][15][16][17] Furthermore, some of these studies examined small or selected populations and ascertaine...