The growing availability of innovative treatments for rare genetic diseases with a cardiac component-such as the mucopolysaccharidoses (MPSs)-has changed these syndromes from 'back of the textbook' curiosities of childhood to chronic, but rare, adult cardiac conditions that require both centres of expertise and knowledgeable subspecialists. The MPSs are inherited progressive lysosomal storage diseases, occurring in about 1:25 000 births and resulting from absence of functional hydrolases responsible for the degradation of glycosaminoglycans, naturally occurring complex sugars ubiquitous throughout the body. In the heart, accumulation of glycosaminoglycans occurs within the cardiac valves, the epicardial coronary arteries, the myocytes and cardiac interstitium and the walls of the great vessels. As a consequence, cardiac valve regurgitation and stenosis, diffuse coronary artery stenosis, myocardial dysfunction and aortic root dilation often occur. Haematopoietic cell transplantation and enzyme replacement therapy have changed the previously lethal natural history of the MPSs to one of survival well into adulthood. Despite this improved lifespan, the left-sided cardiac valves continue to show progressive functional involvement and cardiac valve replacement is not uncommon, especially in adults. The risk of any intervention is increased in these patients because of the systemic effects of the disease on the respiratory system and cervical cord. Our current understanding of other cardiac issues in adults with the MPSs, especially with the coronary circulation and myocardium, is meagre and more needs to be known to effectively care for this emerging population of adults. Incorporation of the MPSs, as well as other nowtreatable rare diseases, into the educational curriculum of current and future adult subspecialists is an important next step.