Previous studies have identified an association between traumatic brain injuries and the development of psychiatric disorders in general. However, these studies were subject to limitations that demonstrate the need for a study of a large, clearly defined mild traumatic brain injury (mTBI) population within an integrated healthcare system. To determine the prevalence and relative risk of postinjury affective disorders over 4 years following mTBI. Cohort study of mTBI cases and matched controls, over a 4-year period. An integrated healthcare delivery system in California. A total of 9428 adult health plan members diagnosed with mTBI from 2000-2007 and enrolled in the year before injury, during which no TBI was ascertained. Control participants included 18,856 individuals selected based on the following criteria: Two unexposed health plan members per each mTBI-exposed patient were randomly selected and individually matched for age, gender, race/ethnicity, and medical comorbidities. Not applicable. A diagnosis of affective disorder (depressive, anxiety, and adjustment disorders) in the 4 years after mTBI or the reference date, determined according to the International Classification of Diseases, Ninth Revision, Clinical Modification as well as the Diagnostic and Statistical Manual of Mental Disorders, 4th Edition, Text Revision. Affective disorders were most prominent during the first 12 months with 23% following mTBI and 14% in the control group. Four-year aggregate adjusted odds ratios for having an affective disorder following mTBI were 1.2 (95% CI: 1.1, 1.2; p < .001) and 1.5 (95% CI: 1.5, 1.6; p < .001) for patients with and without prior affective disorders, respectively. mTBI was associated with a significantly increased risk of having subsequent affective disorders. Screening for and addressing affective disorders at earlier stages following the injury is an important step to avoid persisting conditions that may pose a barrier to full recovery.