Background: We report the case of a 16-year-old male with Mucopolysaccharidosis III type A (Sanfilippo syndrome) who was commenced on risperidone for behaviour management. He rapidly developed extrapyramidal symptoms that have not resolved. Method: The medication histories of 20 patients with Mucopolysaccharidosis III seen at a Lysosomal Storage Diseases Clinic were reviewed to determine the incidence of extrapyramidal side effects. Results: Six patients had been treated with risperidone, olanzapine, or lamotrigine. Five of these patients developed extrapyramidal side effects. Conclusions: The incidence of extrapyramidal side effects was considerably higher than expected. We suggest that these medications be used with considerable caution in these patients. (Contains 1 table.)