During the last twenty-five years it has come to be recognized that a fairly large amount of mental deficiency appears to coincide with or result from faulty intracranial myelination. In some such children little or no post-natal development occurs, in others recession is already occurring at birth; a few remain fairly normal for some years. Spastic contractures, especially of the legs, grow progressively worse, epilepsy may appear, the child goes steadily downhill and early death is the usual termination. In some cases choreoathetosis is present, and this may either, if the patient lives long enough, disappear spontaneously, or, more usually, become impossible when the spastic contractures have immobilized the extremities. Post-mortem one finds one of two conditions—demyelination of the brain (the status dysmyelinatus of Hallervorden-Spatz) or hypermyelination (the status marmoratus of the Vogts). In both forms there is much hypergliosis, evidently secondary to some irritating but slow myelin poison which finally produces anoxia. Clinically during life both varieties appear much the same, and that is all that is known about its nature at present. Cases were recorded by the Vogts in 1920, Hallervorden and Spatz in 1922, Kalinowski in 1927, Helfand in 1931, Spatz and Peters in 1938; in this country by Meyer and Cook in 1936, Meyer and Earl in 1936. By these authors the views of others are summarized—Bielschowski, Loewenberg and Malamud, Urechia and Michalescu, Bouché and Van Bogaert, Casper, Ammosow, Bostroem, Bodechtel and Guttmann. If epilepsy has been present, it generally disappears after some years. By no means all cases end up as helpless idiots; some learn to attend to their own wants, but few ever walk freely.