We describe the distinctive brain CT and MRI findings seen in the putamen of three patients with hyperglycemia. The chief complaint of these patients was either chorea (n=1) or mental change (n=2). They showed hyperglycemia, but physical examination and laboratory data revealed no other abnormalities. In all patients, non-enhanced CT scanning revealed high-attenuated lesions in the unilateral putamen. In two of the three patients, brain MRI performed two days after the onset of symptoms showed an abnormally high signal on T1-weighted images and a low signal on T2-weighted images. One patient had a history of diabetes mellitus, and another had acute myocardiac infarction. The third had no specific history. After the correction of hyperglycemia, the patient's symptoms subsided and diabetes mellitus was diagnosed.