This case report describes two siblings less than 1 year of age who presented severely ill with a dilated cardiomyopathy. Full blood examination in both cases revealed marked granularity of neutrophils suggestive of mucopolysaccharidosis type VI. There were no physical features of a mucopolysaccharidosis but biochemical evaluation confirmed mucopolysaccharidosis type VI in both children. Autopsy in one patient confirmed endocardial fibroelastosis and electron microscopy of fibroblasts in the myocardium showed distention with membrane-bound vacuoles, consistent with a mucopolysaccharidosis. These siblings developed endocardial fibroelastosis before other clinical manifestations of the mucopolysaccharidosis. Assessment for metabolic causes of a cardiomyopathy is important, as cardiac disease may be the initial manifestation of a metabolic disease.