The author states that this thesis is devoted principally to a study of the phenotype of newborn mongoloids. Data obtained by himself from clinical and cytogenetic studies of 38 newborn mongoloid children born during 1961-1962 in southern Sweden are presented. An additional 5 cases of suspected mongolism proved to have normal karyotypes. A further 20 cases of newborn mongoloids born during 1962-63 in the same area were examined cytogenetically by the author, and clinical data from these cases were obtained by questionnaries.