SummaryThe authors reported a sporadic case of adult type of amaurotic idiocy.A female patient who had no specific hereditary taint had well worked as a primary schoolteacher until she married at the age of 27. About the same time she showed slight mental disturbance such as personality change and mild dementia. Subsequently her mental deterioration aggravated, so that she was obliged to retire from school 6 years after the onset of the disease. Then a gait disturbance and the following symptoms successively appeared: muscular rigidity, finger tremor, cerebellar symptoms, myoclonic jerks of the four limbs and generalized convulsions. EEG showed generalized spike and wave complex. The patient died of pneumonia at the age of 42 after the clinical course of 15 years.At autopsy, the brain weighed 900 g. and a moderate enlargement of the lateral ventricles was noted.Histologically the prominent feature was the swelling of the nerve cells of the cerebral cortex except for the area striata. In the regions other than the cerebral cortex, the distention of the neurons was found in the thalamus, oculomotor nucleus, substantia nigra, pontine nucleus, olivary nucleus, Purkinje cells, dentate nucleus and anterior horn cells of the spinal cord.Histochemical examination revealed the storage substance in the nerve cells consisted mainly of phospholipid with PAS‐positive glycolipid. Among the phospholipid sphingomyelin was dominantly exhibite