Diffuse axonal injury (DAI) is a frequent injury after traumatic brain injury (TBI), which causes cognitive and behavioral symptoms. Behavioral changes after DAI affect the patients' quality of life, in addition to causing great damage to their family and society. This study aimed to analyze the behavioral changes of patients with DAI according to family members and to identify the associated factors. This study included patients with DAI, aged between 18 and 60 years, who presented to a referral hospital for traumatic injuries. A prospective cohort study was conducted with 2 evaluations of family members at 3, 6, and 12 months posttrauma. Behavioral changes were evaluated using a questionnaire designed to identify changes according to the perception of family members. The mixed-effects model was applied to identify significant behavioral changes, the effect of time on these changes, and the association between sociodemographic variables, DAI severity, and behavioral changes. Anxiety, dependency, depression, irritability, memory, and mood swings were significantly different (p ≤ .05) before and after trauma. An analysis of the evolution of these behaviors showed that the changes persisted with the same intensity up to 12 months posttrauma. There was an association between depression and income, age and irritability, and DAI severity and dependency. Unfavorable behavioral changes were frequent consequences of DAI, and no improvement in these changes was noted up to 12 months after the injury. Income, age, and DAI severity were related to behavioral changes.