Sanfilippo syndrome: The most common disorder of mucopolysaccharide metabolism, a syndrome in which the onset of clinical abnormalities occurs between ages 2 and 6, with mild coarsening of the facial features (but normal clear corneas), mild stiffening of the joints, slowing of growth, and intellectual deterioration that results in severe mental retardation. On a biochemical level, Sanfilippo syndrome is characterized by the excess excretion of heparan sulfate in the urine and the accumulation of mucopolysaccharides in the central nervous system and other tissues. On the genetic level, there are four types of Sanfilippo syndrome (types A, B, C, and D), each due to deficiency of a different enzyme. All four types are inherited in an autosomal recessive manner and result in identical clinical syndromes. Also known as mucopolysaccharidosis type III (MPS III).