The authors present an intensive study of twelve cases of mental deficiency due to birth injury. They estimate that about 5 per cent of the inmates of the Training School at Vineland are mentally defective from birth injuries. That this etiologic factor in mental deficiency is difficult to determine is shown from their uncertainty as to the etiology in five of the selected cases. In addition to the mental retardation, most of the subjects show speech and motor disorders of varying severity, such as athetosis, spasticity, tremor and incoordination. The number of cases is too small to permit of conclusions as to such problems as the relation of behavior to cerebral integrity, the dependence of intelligence on speech and motor power, the effect of birth injury on intellectual development and behavior, the influence on intelligence of motor reeducation by means of physical therapy and more exact methods of mental testing.