Hypertrophic cardiomyopathy (HCM) is a rare cardiac complication in patients with Klinefelter syndrome. We report the case of a 67-year-old Japanese man with Klinefelter syndrome, HCM, sick sinus syndrome, and coronary arteriovenous fistula, in whom the 47XXY/46XY mosaic pattern was revealed by chromosomal study. Echocardiography revealed HCM with an interventricular septum thickness of 17 mm and a left ventricular posterior wall thickness of 10 mm. Sick sinus syndrome type III was diagnosed by paroxysmal atrial fibrillation (longest sinus arrest 9.0 sec) on 24-h Holter ECG recording. Coronary arteriovenous fistula was detected from the left anterior descending artery to the right ventricle by coronary arteriography. To our knowledge, this is the first case report of Klinefelter syndrome with HCM. As there have been a few reports of patients with Klinefelter syndrome in association with skeletal muscular diseases such as Becker-type muscular dystrophy or myotonic dystrophy, the gene mutation that causes Klinefelter syndrome may occur in the cardiac muscle. HCM may represent another variable expression of this chromosomal abnormality.