This case report describes the history of an 8-year-old boy of full Dutch extraction with mucopolysaccharidosis type III D (Sanfilippo's syndrome type D). Learning problems started at age 5 years; deterioration is only slow at present, and consistent with most of the cases described in literature. Eight of the nine cases described in the literature so far are known to be of Italian origin. Eye investigations showed bilateral peripheral cataract and a fine diffuse white corneal opacity. Ocular abnormalities of this kind have not been reported before in this type of Sanfilippo's syndrome.