<b><i>Background and Purpose:</i></b> Although ataxic hemiparesis is a common lacunar syndrome, the precise mechanism underlying hemiataxia is not clear. We attempted to identify ataxia-related, cerebral blood flow changes in patients presenting with ataxic hemiparesis after acute capsular infarct. <b><i>Methods:</i></b> We used 99mTc-ECD brain perfusion single-photon emission computed tomography to evaluate regional cerebral blood flow in 12 patients with ataxic hemiparesis caused by capsular infarct, and we compared the regional blood flow of these patients with that of 11 patients with pure motor hemiparesis caused by similar lesions. <b><i>Results:</i></b> The ipsilateral red nucleus blood flow was significantly decreased in the ataxic hemiparesis patients, whereas the ipsilateral red nucleus blood flow was increased in the pure motor hemiparesis patients. Crossed cerebellar diaschisis (decreased contralateral cerebellar blood flow) was seen in ataxic hemiparesis patients; similarly, it was seen in pure motor hemiparesis patients. <b><i>Conclusions:</i></b> Our findings suggest that ataxia in hemiparetic patients with capsular infarct can be caused by ipsilateral red nucleus dysfunction secondary to cortico-rubral pathway disruption at the internal capsule. i 2014 S. Karger AG, Basel