Sanfilippo syndrome (Mucopolysaccharidosis III; MPS III) is an inherited lysosomal storage disorder caused by an inability to degrade heparan sulfate. There are 4 types of MPS III (MPS IIIA, MPS IIIB, MPS IIIC and MPS IIID) that are distinguished by the specific enzyme defect. Generally, all types of MPS have similar clinical manifestations although MPS IIIA has been reported to be more severe. Patients usually present in infancy or early childhood with developmental delay, delayed speech, difficulty in feeding, hyperactivity or sleep disturbances. Additional clinical symptoms include intellectual disability, progressive neurologic symptoms including aggressive behavior, poor coordination, seizures and hearing loss. Coarse facies, mild dysostosis multiplex, hepatosplenomegaly and joint contractures may also be present. There is considerable variation in onset and severity of the clinical phenotype even within a sibship. Death usually occurs by the third decade, often due to respiratory complications.