Cardiac involvement has been reported in various mucopolysaccharidoses syndromes. Cardiac valve pathology is the most prominent cardiac manifestation of patients with these syndromes. To date, there have been no reports of early childhood onset of high-grade atrioventricular block in patients with Hunter syndrome. We present a case of a 3-year-old boy with Hunter syndrome who was found to have various degrees of atrioventricular block. This case highlights the importance of early routine cardiac screening for conduction abnormalities and close follow-up in patients with mucopolysaccharidoses syndromes.