Atrophy of the corpus callosum (CC) is a documented consequence of moderate-to-severe traumatic brain injury (TBI), which has been expressed as volume loss using quantitative magnetic resonance imaging (MRI). Other advanced imaging modalities such as diffusion tensor imaging (DTI) have also detected white matter microstructural alteration following TBI in the CC. The manner and degree to which macrostructural changes such as volume and microstructural changes develop over time following pediatric TBI and their relation to a measure of processing speed is the focus of this longitudinal investigation. As such, DTI and volumetric changes of the CC in participants with TBI and a comparison group at approximately three and 18 months post injury and their relation to processing speed were determined.