The appearance of mongolism in both twins is of relatively rare occurrence, and the literature contains only six previously recorded cases. In her excellent monograph on mongolism, Brousseau1 reviewed the literature on mongolism in twins. Recently, Bauer2 made a more detailed and extensive review in which he presented two additional cases and discussed the importance of twin material for the investigation of the etiology of mongolism. To the list of cases reported in Bauer's review must be added additional ones by Waring,3 Novak4 and Petzoldt.5 By a comparison of the reports mentioned, I find that the number of recorded cases of mongolism in one of presumably dizygotic twins is thirty-eight and of monozygotic twins (both being mongolian idiots), seven (including the present case). Six additional cases are. on record in which the information is too scanty to permit classification. This makes the grand total of