A 39-year-old woman with Sanfilippo C syndrome was referred to our department for the treatment of bradycardia. An electrocardiogram revealed a second degree atrioventricular block, and pacemaker implantation was performed with the patient under general anesthesia. A transthoracic echocardiogram showed normal left ventricular systolic function, moderate mitral regurgitation due to mitral valve prolapse, and a high E/e' ratio, indicating left ventricular diastolic dysfunction. The present patient exhibited a rare case of Sanfilippo syndrome complicated with conduction disturbances, mitral regurgitation, and diastolic dysfunction.