Until recently, mild traumatic brain injury (mTBI) or "concussion" was generally ignored as a major health issue. However, emerging evidence suggests that this injury is by no means mild, considering it induces persisting neurocognitive dysfunction in many individuals. Although little is known about the pathophysiological aspects of mTBI, there is growing opinion that diffuse axonal injury (DAI) may play a key role. To explore this possibility, we adapted a model of head rotational acceleration in swine to produce mTBI by scaling the mechanical loading conditions based on available biomechanical data on concussion thresholds in humans. Using these input parameters, head rotational acceleration was induced in either the axial plane (transverse to the brainstem; n=3), causing a 10- to 35-min loss of consciousness, or coronal plane (circumferential to the brainstem; n=2), which did not produce a sustained loss of consciousness. Seven days following injury, immunohistochemical analyses of the brains revealed that both planes of head rotation induced extensive axonal pathology throughout the white matter, characterized as swollen axonal bulbs or varicosities that were immunoreactive for accumulating neurofilament protein. However, the distribution of the axonal pathology was different between planes of head rotation. In particular, more swollen axonal profiles were observed in the brainstems of animals injured in the axial plane, suggesting an anatomic substrate for prolonged loss of consciousness in mTBI. Overall, these data support DAI as an important pathological feature of mTBI, and demonstrate that surprisingly overt axonal pathology may be present, even in cases without a sustained loss of consciousness.