The authors report on a patient with traumatic brain injury who displayed recovery of an injured lower portion of the ascending reticular activating system (ARAS) between the pontine reticular formation and the thalamus. A male patient, age 57, experienced head trauma. After 4 months from onset, he displayed impaired alertness, with a score of 7 on the Glasgow Coma Scale. At 40 mos after onset, the patient had a full Glasgow Coma Scale score of 15. The lower portion of the ARAS was reconstructed using the probabilistic tractography method. On 4-mo diffusion tensor tractography, the fractional anisotropy and the tract volume of the lower portion of the right ARAS were lower by more than 2 SDs of those of control subjects. By contrast, on 40-mo diffusion tensor tractography, the fractional anisotropy and the tract volume values of the lower portion of the right ARAS were within 2 SDs of those of the control subjects. The increment fractional anisotropy and the FV value of the lower portion of the right ARAS on 40-mo diffusion tensor tractography indicated recovery of the injured lower ARAS. Because of this, recovery of an injured lower portion of the ARAS in a patient with traumatic brain injury was demonstrated.