A cross-sectional survey of ten physical measures was carried out on 2,472 institutionalized mentally defective patients. This report gives the results in the Caucasian segment numbering 2,027. The means and standard deviations for each measure are shown by sex, age, I.Q. level, and diagnostic category. Body weight, crown-heel height, symphysis-heel height, biacromial diameter, and bicristal diameter had smaller dimensions than normal in both sexes and the degree of impairment related to the degree of I.Q. deficit. No differences among diagnostic categories in degrees of stunting were noted with the exception of Mongolism in which stunting was greater than in the other categories. Shortening of the lower extremities accounted for this difference. In contrast, the measurements of the width of the shoulder and pelvic girdles and length of the spine showed in mongolism no differences from the remaining diagnostic categories. The literature on growth effects of experimental brain lesions was reviewed. These reports indicate that various known physiologic mechanisms may be involved especially after certain hypothalamic and amygdaloid lesions. Further work is needed to elucidate the effect of brain lesions on the growth-hormone mechanism. On the basis of the experiments with brain lesions and observations in the human, it is concluded that the way in which brain injury produces stunted growth in the human is unclear. Further work will be needed on genetic, nutritional, neurophysiological, and endocrine aspects of physical growth before this mechanism can be defined.