How can we identify traumatic brain injury (TBI) patients at risk for long-term brain injury? In this longitudinal study, 57 patients with a relatively good clinical status on admission underwent MRI within 48 hours and at 90 days after injury. Brain volume changes were markedly larger in patients with subarachnoid hemorrhage (-3.2%) compared to patients without subarachnoid hemorrhage (-0.4%; P <0.001). Perfusion was moderately correlated with brain volume change at 90 days (ρ = 0.39; P = 0.003). This demonstrates the utility of imaging markers on acute MRI, especially subarachnoid blood, to identify patients at risk for long-term brain injury.