A Japanese boy with the typical manifestations of 18q-syndrome and del
ayed myelination on magnetic resonance imaging is described. Cytogenet
ic investigation revealed a deletion at 18q21.3. Three serial magnetic
resonance images demonstrated that myelination in the central nervous
system was delayed except for the corpus callosum and brainstem. This
pattern of delayed myelination appears to be peculiar to the 18q- syn
drome. Because the gene for myelin basic protein has been localized to
the distal end of the long arm of chromosome 18, we speculate that th
e abnormal myelination in our patient was partly due to the failure of
expression of the myelin basic protein gene.