The morphological changes in the functional structure of the placenta in cases of idiopathic intrauterine growth retardation (IUGR) have been investigated with quantitative analyses. It has shown that the placentae of the small-for-gestational-age (SGA) infants were 45 per cent smaller than the controls, due to an almost equal reduction in parenchymal (48 per cent) and non-parenchymal (42 per cent) tissues. Consequently, in relative values the placentae of the SGA infants were shown to be otherwise very similar to the controls. In contrast, in absolute values, the placentae of the growth-retarded infants were shown to differ from the controls by having significantly less surface area of exchange between mother and fetus, mainly in terms of peripheral capillary (3.85 vs 7.21 m2; P less than 0.02) and villous (4.31 vs 8.74 m2; P less than 0.005) surface areas, and intervillous space volume (53.71 vs 103.26 cm3; P less than 0.001). The total number of cells (53 418 x 10(6) vs 104 820 x 10(6); P less than 0.005) was also shown to be markedly decreased in the placentae of the SGA infants, compared with the controls, and the placental membrane showed a 51 per cent decrease in trophoblast mass or cellular content. On a functional basis, these findings suggest that, in these cases of idiopathic IUGR, the placental morphological changes are bound to produce placental insufficiency and fetal growth retardation.