MRI was performed in two siblings with the neuropathic sphingomyelinase deficiency caused by identical mixed heterozygosity in the structural acid sphingomyelinase gene. The clinical phenotype of the cases is unique in showing a rather protracted course, both having reached the fourth decade. Pronounced cerebellar and mild supratentorial atrophy was seen on MRI in both siblings, in contrast to their strikingly different clinical status. One has no overt neurological deficit, while the second had neocerebellar symptoms and signs, nystagmus and cranial nerve palsies for some years. The MRI findings, together with the cherry-red spot in the ocular fundus, ultrastructurally proved storage in cutaneous nerve Schwann cells and the histopathologically proven brain neuronal storage in a third sibling who died after a relatively rapid course dominated by fatal visceral storage, is evidence that a remarkably restricted neuropathology can be caused by this enzymopathy.