A quantitative study has been made of organ and cellular development in eight newborn and seven older infants with congenital rubella. The infants were subnormal in size at birth due to retarded intrauterine growth. A further retardation of growth was observed in the postnatal period. The basis of growth retardation in such infants is a subnormal number of cells in many body organs. The cause of the growth disturbance appears to be an activity of the proliferating virus which inhibits cell multiplication. Necrotic or inflammatory changes in the tissues are rare. The subnormal number of cells in many body organs provides a partial explanation for the long-term physical and mental retardation encountered in some persons who have had congenital rubella.