1. In order to study thyroid and lipid function in mongolism, the levels of serum protein-bound iodine, cholesterol, and the fraction of Sf 12-20 molecules of lipoproteins were studied in a group of 74 mongoloid patients, and these were compared with the findings in normals and with a group of 18 cases diagnosed as "undifferentiated mental deficiency."2. There was no significant difference in the serum protein-bound iodine levels between mongoloid children and "controls" of the same age.3. Mentally retarded children, whether the diagnosis is mongolism or undifferentiated mental deficiency, have significantly higher serum cholesterol levels than normal children.4. The most marked differences between mongoloids, normal, and control children occur in the level of large molecule lipoproteins of the Sf 12-20 class, the mongoloids being highest, the cases of undifferentiated mental deficiency intermediate, and the normals lowest.Abstract Teaser