A 68-year-old woman was admitted to our hospital because of sudden onset of involuntary movements, similar to those associated with chorea, of the right side of the body and for further evaluation of thrombocythemia. She had no family history of chorea. Neurological findings did not show any abnormality except for chorea of the right side. Laboratory studies showed increased number of white blood cells (14,000/microl) and platelets (188.3 x 10(4)/ microl). Lupus anticoagulant, anti-cardiolipin antibody, and ceruloplasmin levels were within the normal range. Her NAP score was 240, and result for bcr-abl gene expression was negative. Bone marrow puncture showed hypercellularity and increased number of megakaryocytes (550/microl), but there was no atypism. On the basis of these laboratory findings, she was diagnosed with essential thrombocythemia T1-weighted magnetic resonance imaging (MRI) showed a hyperintense lesion extending from the region around the left globus pallidum to putamen. The MRI findings of our study were similar to those related to diabetic hemichorea; however, the results of some tests did not indicate diabetes mellitus. An MRI scan showing high T1 signal intensity in the basal ganglia might not be specific for diabetic hemichorea. In this case, MRI revealed the cause of hemichorea to be micocirculatory failure or small cerebral hemorrhages.