The various disorders caused by heritable defects in complex carbohydrate catabolism comprise two groups: (A) The mucopolysaccharidoses, six main and several subtypes are described. (B) The mucolipidoses (oligosaccharidoses), at least nine types being recognized. Whilst most of these are now well defined by clinical and biochemical studies, much of the sequence of events from the intrinsic metabolic error to their clinical features remains obscure. Most are transmitted as autosomal recessive conditions, a mode of inheritance often, as with these disorders, associated with enzymic deficiencies. All patients display the Hurler phenotype, but this, as also the characteristic bone changes, varies widely in severity both within and between the specified types of disease. The radiological abnormalities--dysostosis multiplex--indicate the broad disease complex and are rarely type-specific; diagnostic precision needing knowledge of both clinical and biochemical examinations. In several types mental development is normal and bone changes are mild, so permitting confusion with other forms of bone dysplasia or disease.