Ferritinopathy (neuroferritinopathy) has recently been identified as an autosomal dominant, multisystem disease, mainly affecting the central nervous system. It is caused by mutations in exon 4 of the ferritin light chain gene on chromosome 19. Its fine structural hallmarks are granular nuclear inclusions in neurons, oligodendroglial and microglial cells with similar extracellular derivatives in the central nervous system, muscle, peripheral nerve, and skin. These pathognostic structures have previously been described in perivascular cells of muscle and nerve biopsy specimens in a case with an obviously identical disease, formerly described as 'granular nuclear inclusion body disease'. The nuclear inclusions, at the light microscopic level, are iron positive following histochemical iron reactions and immunoreactive for ferritin antibodies. At the electron microscopic level, in contrast to filamentous nuclear inclusions in 'neuronal intranuclear hyaline inclusion disease', dominant spinocerebellar atrophies and other trinucleotide repeat diseases, they are basically composed of granules measuring 5-15 nm. A moderate peak of iron detectable by energy dispersive microanalysis of the granular nuclear inclusions in ferritinopathy may also be significant. It is emphasized that ferritinopathy or 'granular nuclear inclusion body disease' can be diagnosed by a simple muscle or nerve biopsy without brain biopsy, autopsy, or molecular genetic testing of the considerable number of neurodegenerative diseases with possibly similar symptomatology.