MLS (Maroteaux-Lamy syndrome) or MPS VI (mucopolysaccharidosis VI) is an autosomal recessive pathology in which there is absence or low activity of the enzyme N-Acetylgalactosamine-4-Sulfatase, which hydrolyzes GAGs (glycosaminoglycans) in the body (mainly dermatan sulfate). Consequently, there occurs lysosomal deposition of GAGs in connective tissue multisystemic. Myocardium and heart valves are frequently affected structures, presenting a direct correlation with the reports of complications and deaths. Case report: RGS, male, 3 years and 2 months, diagnosed with MPS VI from the first month of life, in weekly ERT (enzyme replacement therapy) since 4 months of age (inconstant). At physical examination: normotensive, with holosystolic heart murmur 3+/6+ in mitral focus. Complementary tests: normal electrocardiogram, echocardiogram with pronounced mitral regurgitation, concentric left ventricular hypertrophy of moderate degree and mild aortic insufficiency. Discussion: Mitral valve disease is common in patients with MLS. Conditions such as cardiomyopathy, fibroelastosis, aneurysm and pulmonary hypertension may occur in these patients, indicative of morbidity and mortality. Early and constant ERT may be useful in slowing a progression of heart disease. Conclusions: follow-up with a cardiologist is important to evaluate the progression of cardiac complications in MPS VI. Constant and early ERT provides better prognosis for these patients.