Some cases of hemichorea-hemiballism (HCHB) are associated with a hyperintense putamen on T1-weighted MR images, the cause of which remains unclear. Our purpose was to determine the cause and significance of these MR signal changes. We analyzed the clinical and neuroimaging findings in 10 patients with HCHB, focusing on locations of the hyperintense lesions on T1-weighted images, comparing them with those on CT scans, and evaluating their changes after years of follow-up. A biopsy was performed in one patient. Seven patients had hyperglycemia and two had cortical infarcts. HCHB recurred in four patients. A hyperintense putamen preceded the occurrence of HCHB in two patients. T1-weighted MR images revealed hyperintense lesions limited to the ventral striatum in six patients. Hyperintense lesions extended to the level of the midbrain in one patient and persisted for as long as 6 years in another patient. T2-weighted MR images revealed slit-shaped cystic lesions in the lateral part of the putamina 2 to 6 years after the onset of symptoms in two patients. A biopsy specimen from the hyperintense putamen in one patient revealed a fragment of gliotic brain tissue with abundant gemistocytes. Proton MR spectroscopy of the specimen showed an increase in lactic acid, acetate, and lipids, and a decrease in N-acetylaspartate and creatine, suggesting the presence of pronounced energy depletion and neuronal dysfunction. Gemistocytes are sufficient to explain the shortening of T1 relaxation time. Our investigation suggests that neurons in the ventral striatum and striatonigral pathway may play a critical role in generating ballism.