Background: Demonstrating the precise anatomical area of the internal border zone (IB) by brain imaging has been unclear, and it is not well known which relevant arteries are associated with IB infarction. Methods: Patients with IB infarctions were selected from 748 consecutive patients with acute ischemic stroke. The IB infarctions were identified by coronal diffusion-weighted imaging, excluding lesions over the upper pole of the lateral ventricle. The angiographic findings of the internal carotid artery (ICA) and middle cerebral artery (MCA), on contrast-enhanced MRA, were evaluated in the patients with and without IB infarction. Results: Thirty patients met the criteria for an IB infarction: 7 had MCA steno-occlusion without ICA disease, and 23 had ICA steno-occlusion. Sixty-one patients had ICA steno-occlusion without IB infarction. The multiple logistic regression analysis showed that a more than moderate degree of steno-occlusion of the MCA was a significant factor (OR, 11.32; p = 0.006) associated with IB infarction; whereas that of the ICA was not significant (OR, 2.19; p = 0.298). Conclusion: The results of this study suggest that IB infarctions were associated with MCA steno-occlusion. ICA disease resulting in IB infarctions would be expected to have significant MCA steno-occlusion causing hemodynamic compromise.