Fifteen light for dates infants and their placentae were compared to 15 well-grown infants and their placentae. The former were born to thin, underweight women while the latter were born to women of normal weight. The light for dates infants were symmetrically growth retarded but not wasted at delivery and their placentae had a reduced weight, volume, chorionic surface area, percentage parenchyma and total villous surface area. The peripheral villous surface area and volume of peripheral villous trophoblast, fetal capillaries and connective tissue was also reduced in the placentae of light for dates infants, suggesting retarded placental growth in the latter half pregnancy. In contrast, the stem villous surface area and volume of stem villous trophoblast, fetal capillaries and connective tissue was similar in both groups of placentae, suggesting the same rate of growth in early pregnancy. There were no differences in the volume of fibrin or infarcts. The ratio of total villous surface area to infant weight, length and head circumference was reduced in the light for dates infants. This may restrict the materno-fetal oxygen exchange, and thereby increase the risk of fetal hypoxia during labour. It is concluded that the placentae of light for dates infants born at term to underweight women are both absolutely and relatively small with a reduced villous surface area.