120 children in a school for mentally retarded children with an IQ below 75 were examined. 40% of them were firstborn, 27% second, 12% third children. 19% were born of consanguineous parents (in comparison with 4% only in a "normal school" in Jerusalem). 11% had fathers and 5% mothers over 40 years of age. At birth they weighed less than other babies, also their length was smaller. 51% boys had subnormally developed external genitals. There is no difference in the percentage of lefthandedness between deficient and normal children in Jerusalem. More than a half of the deficient children showed microcephaly, 12% showed mongolism, 6% suffered from epilepsy, and 5% from congenital cerebral infantile paralysis. In 30% of all cases deficiency was due to external causes (16% due to difficult birth; 14% deficiency acquired after birth, e.g. encephalopathy particularly following whooping-cough.) Endocrine therapy is of no avail in genuine mental deficiency; research, however, has led to a more effective treatment, resulting in relative improvement of infantile deficiency.