One child in every 700 born in Britain today is a mongol. Mongolism is a common form of mental deficiency presenting a picture of delayed physical and mental development. Recent research suggests that this condition is caused by a defect in the chromosome balance within the human cell. The significance of this and the well-being and upbringing of these children is discussed by a consultant psychiatrist, L. S. Penrose, M.D., F.B.S., Professor of Eugenics, University College, London and a Medical Superintendent of a Mental Deficiency Hospital.