We report 3 diabetic patients who developed hemiballism without involvement of the subthalamic nucleus. Each patient exhibited vigorous, flinging, ballistic involuntary movements in the extremities and slight facial grimacing involving one side of the body. Although diabetes was poorly controlled in all 3, each patient was nonketotic at the onset of hemiballism. Magnetic resonance imaging (MRI), in these patients showed abnormalities in the striatum contralateral to the hemiballism that were characterized by an increase in intensity on T1-weighted images and a slight decrease in intensity on T2-weighted images, and these changes persisted for more than 2 months. The striatal lesions are presumed to have developed following mild ischemia in the territory of the lateral striate branches of the middle cerebral artery. This combination of hemiballism and striatal lesions in diabetic patients may constitute a unique syndrome.