A 47-year-old male was referred to our hospital to undergo examinations for male infertility and was diagnosed with Klinefelter's syndrome (47XXY). He was subsequently referred to our institute due to the occurrence of involuntary movements in his left lower extremity and a high FPG level of 576 mg/dl. He suffered from ballism in the left lower extremity, and his HbA1c level was 11.8 % (NGSP value). He was diagnosed with both diabetes and renal dysfunction. Hypoplasia of both kidneys and hepatocellular carcinoma were detected on abdominal CT, while hypoplasia of the left vertebral and right internal carotid arteries was observed on neck MRA. Intensive insulin therapy was initiated, and the symptoms of ballism rapidly disappeared in association with normalization of the blood glucose level. Diabetes occurring concomitantly with Klinefelter syndrome is observed at a comparatively high rate of 15-50 %. The cause of ballism in this case was suspected metabolic aberration due to prominent hyperglycemia in addition to cerebral tissue ischemia resulting from hypoplasia of the blood vessels and dehydration.