THIS paper reports the results of treatment of patients with congenital hypothyroidism over a period of thirteen to twenty-three years. Emphasis has been placed on their mental development and on their physical, behavioral, educational and socioeconomic progress. In the treatment of a patient having a chronic disorder, an important consideration is his restoration to maximum usefulness so that he may take his proper place in society. In congenital hypothyroidism, for which specific therapy is available, it would appear that this goal should not be difficult to accomplish. As far as the patient's physical development is concerned, such a result is usually achieved. However, it is the opinion of many investigators that even after prolonged and apparently adequate treatment the mental attainments of the congenitally hypothyroid patient frequently leave much to be desired. This view was supported by Bruch and McCune1who after intensive longitudinal studies on 23 children with