Fornix-to-brain ratios (FBR) based on postinjury magnetic resonance (MR) studies were calculated on a group of 27 female traumatic brain injury (TBI) patients and 18 female medical controls by taking the widest aspect of the fornix at the level of the anterior horns and third ventricle and determining a fornix surface area. The FBR was significantly reduced in the TBI group (FBR = 0.1121) as compared to the normal control group (FBR = 0.1766). Despite these significant FBR findings indicating prominent atrophic changes of the fornix in TBI patients, the FBR did not relate systematically to neuropsychological outcome. These findings clearly indicate fornix vulnerability in TBI and that current quantitative MR methods are sensitive enough to detect such changes. However, fornix degeneration constitutes only one of many contributing factors to the anatomic basis of TBI-induced cognitive disturbances, as fornix atrophy did not relate systematically to neuropsychological outcome.