There are between five and ten thousand Mongolian idiots in Germany and for their families the question of heredity is a question of 'to be or not to be'. The author investigated thirty-four cases of Mongolian idiots and their families. In his conclusions he denies the theory of heredity and summarizes his convincing material supporting the thesis that mothers of Mongolian children suffer from ovarian disturbances. Even the concordance of identical twins cannot contradict this statement for apparent reasons. Mongolian idiocy is caused by a 'dysplasmotic ovulum'. The