Six placentae from small for gestational age infants were examined by both light and electron microscopy. These were from pregnancies in which all maternal or fetal factors known to be associated with intrauterine growth retardation, including maternal cigarette smoking, were excluded. At the light microscopic level the only significant finding was an excess of villous cytotrophoblastic cells whilst electron microscopy showed these placentae to be characterized by villous cytotrophoblastic hyperplasia, focal syncytial necrosis, microvillous abnormalities, reduced syncytial secretory activity, irregular thickening of the trophoblastic basement membrane and the presence of small fetal villous vessels with multilayered basement membranes. It is thought that most of the observed abnormalities are due to uteroplacental ischaemia and it is possible that the fetal vascular abnormalities are a reflection of the fetal growth retardation. There is little evidence that the functional efficiency of the placenta is impaired in these cases and it is suggested that the principal factor in the pathogenesis of fetal growth retardation is a restriction of nutrient supply to the fetus because of an inadequate degree of physiological change within the maternal spiral vessels.