This article contrasts 2 groups that sustained somewhat similar moderate to severe closed-head traumatic brain injury (TBI), but were deliberately selected to be different with regard to postinjury intellectual status--one group average or above, the other below. The purpose of this comparison was to describe any morphological characteristics of the 2 groups ascertained from quantitative magnetic resonance (QMR) imaging. Thirty-five TBI participants with Full Scale IQ (FSIQ) less than or equal to 90 were compared to 33 TBI participants whose FSIQ was above 90. A group of normal volunteer participants, age and gender matched, constituted a third magnetic resonance comparison group. All participants received uniform MRI from which QMR analysis was performed, including total cranial volume, subarachnoid cerebral spinal fluid ventricular volume, and hippocampal volume. Both TBI groups received neuropsychological testing in the course of clinical follow-up. Morphological comparisons between groups were made using multivariate analysis of variance. The TBI group with an IQ less than or equal to 90 had significantly enlarged third and temporal horn compartments. Total intracranial volume was smaller in this group as well. Lower psychometric intelligence postinjury may be associated with more temporal lobe atrophy and subcortical pathology. Smaller premorbid brain size may be another risk factor.