Mild Sanfilippo syndrome: a further cause of hyperactivity and behavioural disturbance.

Sanfilippo syndrome (mucopolysaccharidosis type III) usually presents with severe behavioural disturbance and it has characteristic features. The patient's condition follows a neurodegenerative course; death generally occurs in the second decade. Five patients, from three families, who presented with a mild variety of the syndrome are reported here. Four of the five patients did not show the usual somatic features, and none showed regression. It is important that clinicians be aware of this mild variant so that appropriate tests can be undertaken in retarded children and adults.