We report the case of a 31-year-old man who had mild traumatic brain injury as a result of an accident at the age of 24 years. Seven years after the trauma, at the age of 31 years, he had a lower verbal intelligence quotient than performance intelligence quotient by the Wechsler Adult Intelligence Scale - Revised, and frontal lobe dysfunction, for example, difficulty in maintaining or changing the set as revealed by the Wisconsin Card Sorting Test Keio Version. Conventional brain magnetic resonance imaging had not shown any abnormalities. Abnormal brain areas were detected on magnetic resonance diffusion tensor imaging. On tractography, some fibres from the corpus callosum towards the frontal cortex were noted to be lacking in the left hemisphere compared with the right. The tractography results may explain the patient's lowered verbal intelligence quotient and focal left frontal lobe dysfunction. Diffusion tensor imaging is therefore helpful in detecting lesions in mild traumatic brain injury with diffuse axonal injury.