Because of the importance of the serum lipids in the pathogenesis of atherosclerosis, much information is available on the serum lipoproteins of adults in health and disease (Adlersberg et al., 1955; Chapin, 1956; Dangerfield and Smith, 1955; Jencks et al., 1956; Kanabrocki et al., 1958). The lipoproteins in childhood have been studied less extensively. Rafstedt (1955) has reported lipid and lipoprotein values for normal infants and children. More recently, Salt and Wolff (1957) have found that paper electrophoresis of the lipoproteins is of value in a number of diseases of infancy and childhood involving lipid metabolism primarily or secondarily. These authors detected abnormalities in the serum lipoproteins in gargoylism and the severe form of hypercalcaemia of infancy, conditions associated with mental defect. Using the ultra-centrifuge, Simon et al. (1954) had earlier demonstrated an increase in the level of certain low density lipoproteins in patients with mongolism, and work in this department (Stern and Lewis, 1957, 1958a) indicated that lipid metabolism is disturbed in children with mongolism and some other mentally retarded children. Paper electrophoresis was chosen as the method for a further study of the serum lipids in mental deficiency. Of particular interest was the possible relation of abnormalities in the serum lipoproteins to the disturbance responsible for the mental defect.