Early post-traumatic seizures occur commonly and may have adverse clinical consequences. In order to determine the significance of post-traumatic seizures, we performed a prospective assessment of the consequences of epileptic activity by assessing the change in extracellular glycerol levels. Glycerol is a marker of cellular membrane breakdown. Thirteen patients underwent combined electroencephalography (EEG) and cerebral microdialysis monitoring. Two patients had seizures on EEG with associated delayed elevations of glycerol associated with the seizure activity. Higher mean levels of glycerol were present in those patients with seizures compared to those without seizures (p < 0.001). Preliminary evidence suggests that post-traumatic seizures lead to additional membrane injury as reflected by elevated extracellular glycerol levels.