Characteristic changes in the MRI of 2 brothers with mucopolysaccharidosis type I are reported from the Departments of Paediatrics, Neuroradiology and Neurology, University of Ancona, Italy. One patient was mentally retarded and the other had a normal IQ. The MRI showed a honeycomb-like appearance of the thalami, hypoplasia of the cerebellum and an enlarged ventricular system, with changes in the white matter behind the occipital horns. These changes were not correlated with the mental retardation. 
COMMENT. Glycosaminoglycans and glycolipids accumulate in the cells of the central nervous system in mucopolysaccharidosis type I. It is suggested that the mental retardation could be tied to neuronal damage caused by lysosomal storage of these substances. The cell biology of glycosaminoglycans in regulating normal cerebrospinal fluid absorption is reviewed by Knepper PA and McLone DG, Children’s Memorial Hospital and Northwestern University Medical School, Chicago.